What Is Image SEO?
Images are the single biggest cause of slow pages and the single most neglected content asset on most sites. Those are two unrelated problems with different answers. Treating them as one subject is why most image advice is a list of tips.
Two Separate Problems
Images make pages slow. Images are content nobody describes. Those are different failures with different fixes. Combining them into one subject called image optimisation is why the advice usually arrives as an undifferentiated list.
The weight problem. Technical and solvable.
Photographs are enormous, pages carry many of them, so the result is slowness. Fixable once, mostly by decisions rather than by ongoing effort.
The content problem. Editorial and ongoing.
Images carry information nothing describes, so nothing can find them or use them. That needs writing rather than configuration, which is why it never gets done.
Why separating them matters. Different people fix them.
The first is a developer or a platform setting. The second requires somebody who knows what the photograph shows. Treating them as one task assigns both to whoever is available.
Why both get neglected. Nothing complains.
A slow page still loads and an undescribed image still displays. Neither failure produces an error or a warning, so both persist on sites that look and behave as though nothing is wrong.
Which is more valuable. Depends on the business.
For a text-heavy site, the weight problem. For a business whose work is judged visually, the content problem is worth considerably more, which is block seven.
A note on this page. It shows no images.
Our guides carry one image in the hero and nothing below it, which is the standard format across this whole section. On a page about images that reads oddly. It is deliberate rather than an omission.
Compression And Formats
Original photographs are enormous. A modern phone produces files far larger than any web page needs. Uploading them untouched is the commonest cause of a slow site we encounter.
What compression does. Removes data.
Reduces the information in a file, either without visible change or with a small loss most people never notice. The second is almost always the right choice for photographs on a website.
Why the saving is so large. The original is for printing.
Cameras capture detail suitable for reproduction at size. A photograph displayed in a browser needs a fraction of that. The difference is not subtle.
The format decision. Newer formats are smaller.
Modern image formats produce noticeably smaller files at the same visible quality than the ones most sites still use. Support is now broad enough that this is a straightforward choice.
Why we state no target size. It depends entirely.
A full-width photograph and a small logo have nothing in common. Any threshold would be wrong for most images on a page. It would also invite optimising towards a number.
What most platforms do now. Handle it.
Modern hosted platforms compress and convert on upload without being asked. If yours does, this whole block is already solved and the effort belongs elsewhere.
Serve The Size You Display
A large image scaled down in the browser still downloads at full size. The page looks correct, the image looks right, then the visitor has waited for data that was discarded before display.
What actually happens. Download, then shrink.
The browser fetches the whole file and then draws it smaller. Nothing about the visual result reveals that, which is why this survives on sites that look perfectly built.
Why it is so common. Scaling is invisible.
A photograph inserted at whatever size it arrived will be constrained by the layout automatically. It looks intentional, while nobody resized the underlying file.
Where it hides. Thumbnails and galleries.
A grid of small images each downloading a full photograph is the worst case, because the count multiplies the cost. Galleries are frequently the heaviest part of a site.
Why this matters more than compression. The multiple is larger.
Compression reduces a file somewhat. Serving the displayed size can reduce it by an order that compression cannot approach. The two work together.
What the fix is. A platform behaviour.
Modern systems generate several sizes and serve the appropriate one. This is a setting or a platform capability rather than something to do image by image.
Alt Text
Alternative text exists for people who cannot see the image. That is its purpose rather than a search device. Writing it properly for that purpose happens to serve everything else as well.
Who reads it. Somebody using a screen reader.
Software reads the description aloud in place of the image. Without one, a person hears nothing where a photograph is, else hears a filename, which is worse than nothing.
Why the order matters. It changes what you write.
Somebody writing for a search engine produces a phrase. Somebody writing for a person produces a description. The second is more useful to both, which is why the framing is not merely polite.
What else uses it. Several things.
Search engines assessing what an image shows, browsers where an image fails to load, plus anything converting a page to another format. All served by an accurate description.
Why it is neglected. Nobody sighted encounters the gap.
A page with no alternative text looks identical to one with it. There is no visible fault, no error and no prompt, which is the same pattern as broken heading structure.
What the obligation is. More than search.
Describing images is an accessibility requirement rather than an optimisation. That reason holds regardless of what any search engine does with the text.
How To Write It Properly
Describe what the image shows, in the context of the page it sits on. Stuffing it with phrases is useless and a genuine disservice to the person it exists for. Decorative images need none at all.
The test. Would it replace the image.
Read your description instead of looking at the photograph. If somebody would understand what they are missing, it works. If they would learn nothing, rewrite it.
Why context matters. The same photograph varies.
A picture of a finished wall on a services page and on a case study need different descriptions, because what matters about it differs. The image is the same and the useful description is not.
What stuffing does. Two harms at once.
It achieves nothing for search and it produces a stream of repeated phrases read aloud to somebody who wanted to know what the picture showed. That is the disservice.
Decorative images. Leave them empty.
Background textures, dividers and purely visual elements should carry no description, so assistive software skips them. Describing decoration is noise rather than helpfulness.
How long to spend. Seconds each.
A short accurate sentence is the whole requirement. Anybody agonising over alternative text has misunderstood what it is for.
File Naming
A descriptive filename is free on a new image and marginal in effect. Renaming an existing library is rarely worth the effort. We would talk a client out of paying for it.
What a good filename looks like. Words, not codes.
Something describing the subject rather than a camera reference number. Costs nothing at the point of upload, because you are naming the file anyway.
What it contributes. A little.
It is one weak indication of what an image shows, alongside the description and the surrounding text, which are both stronger. Useful and not decisive.
Why renaming is rarely worth it. The addresses change.
Renaming files on a live site changes where they live, which means every reference has to be updated. That is real work for a marginal gain.
The exception. A rebuild.
Where a site is being rebuilt or migrated anyway, naming things properly costs nothing extra. That is when to do it, rather than as a task of its own.
Why it appears on every checklist. It is easy to check.
Filenames can be inspected and reported quickly, which makes them an appealing finding. Appearing in a report is not the same as mattering.
Images Are Content Nobody Writes About
A photograph with words around it can be found. A bare gallery cannot. For businesses whose work is judged by how it looks, that distinction is worth more than everything else on this page.
What a bare gallery is. Pictures, nothing else.
A grid of photographs of completed work, no text, no context, no explanation. Common, understandable and invisible to anybody who is not already on your site.
Why it cannot be found. Nothing describes it.
There is no text stating what the work was, where it was, what the problem was or how it was solved. There is nothing for a search to match.
What the alternative looks like. Photographs with an account.
The same images with a few sentences each: what this was, what the difficulty was, what was done. That converts a gallery into pages that can answer searches.
Where this matters most. Visual trades.
In our plastering research, people searched to see a particular finish before choosing anybody. Somebody wanting to see a specific type of work is looking for pictures with words attached.
Why nobody does it. It is writing.
Uploading photographs takes minutes. Writing three sentences about each takes longer and feels less productive, so the gallery gets built and the words never arrive.
What the words should cover. The problem, not the picture.
Nobody searches for a photograph of a wall. They search for the problem the wall had, so the useful text describes the situation somebody arrived with rather than restating what the image already shows.
What to do about a backlog. The best jobs only.
Nobody is going to write up two hundred photographs. Ten jobs described properly is worth more than a gallery of two hundred nobody can find.
Captions Do More Than Alt Text
A caption is read by people who can see the image. Alternative text is read by people who cannot. They serve different audiences. Most sites have neither.
What a caption is for. Adding what the picture omits.
The photograph shows a finished surface. The caption says where it was, what was underneath and how long it took. That is information the image cannot carry.
Why they are read. Unusually reliably.
People scanning a page look at images and then read the text immediately beneath them. A caption sits in one of the few places on a page that is genuinely read.
How they differ from descriptions. Assumed sight.
Alternative text substitutes for the image and should describe it. A caption accompanies the image and should add to it. Duplicating one as the other wastes both.
What they contribute to search. Real text on the page.
Captions are ordinary content, unlike alternative text which sits in an attribute. That makes them the more substantial of the two for a page trying to be found.
When to skip them. Obvious images.
A photograph whose subject and relevance are plain from the surrounding text needs nothing added. Captioning everything produces repetition. The value is in the images where context is genuinely missing.
Why they are rare. No field prompts them.
Most editors ask for alternative text and treat captions as optional styling. The interface decides what gets written, which is a recurring pattern in this section.
Our Own Rule On Filenames
We never put image filenames into structured markup. That has governed every page in this programme, including this one. The reasoning is worth setting out because you can verify it.
What the rule prevents. Brittle declarations.
Markup naming a specific file breaks silently when that file is renamed, moved or replaced. The page continues working and the declaration is now wrong.
Why that matters more than it sounds. Nobody checks.
A broken reference inside markup produces no visible fault. It sits there indefinitely, asserting something untrue about a page nobody has reason to inspect.
What we do instead. Describe the page.
Our markup describes what a page is, who wrote it and what it covers. Those things do not change when a designer replaces a photograph.
Why we are telling you. It is inspectable.
You can look at this page's markup and confirm no filename appears. That is a stronger form of advice than asserting a practice.
The general principle. Declare durable things.
Anything in markup should be something that stays true. Optimising existing content applies the same reasoning. User behaviour covers the accessibility argument again. The full series is on the on page SEO guide.
Ten jobs described
beats a gallery
of two hundred.
A photograph with words around it can be found. A bare gallery cannot, because nothing states what the work was, where it was or what the difficulty was. Uploading images takes minutes and writing three sentences about each does not, which is why the words never arrive.
What we check on images:
We would talk you out of paying to rename an existing image library.
Every guide.
One practice.
What a page says about itself, headings, keywords, search intent, images, page types, user behaviour, credibility and how to review an existing site.
