Image SEO: alt text, filenames and context

What affects image SEO

An image enters search results when the engine can understand what it shows and where it lives. The strongest signals are text:

1. The page is indexable and the page content matches the image.
2. The image has descriptive alt text.
3. The filename says something concrete.
4. The surrounding caption and heading repeat the subject.
5. The image is registered in an image sitemap (see the image sitemap reference) so discovery is fast.

A common misconception is that keywords in image names carry a direct rank boost. Their effect is through relevance: an engine matching a query to chunky-knit-wool-jumper is more confident than to IMG_2041.jpg. The same logic applies to alt text.

Alt text: write for the user first

Alt text is the most important single factor. Its primary purpose is accessibility: a screen reader converts it to speech, and a broken image displays it instead of a blank. It also makes the image interpretable for the engine. Rules that cover both:

1. Describe what the image shows in plain words.
2. Never spam keywords into alt. If the page already says "bespoke site design", repeating it adds no search value and reads badly to a reader.
3. Leave alt="" for decorative images; they should not claim content.
4. Do not duplicate the surrounding paragraph verbatim.

Before and after:

Filenames

Give every file a concrete name before upload:

• Use descriptive words separated by hyphens: bespoke-web-design-cape-town.jpg.
• Avoid IMG_, dates, and random strings; they carry nothing.
• Reuse the same words as the alt text; the pairing is consistent for both humans and machines.
• Keep filenames short but specific: turtle-sa-coast.jpg beats turtles-and-sa-coastlines-photography.jpg.

Context and supporting markup

The page frame matters as much as the tag:

• Use a real caption (<figcaption>) beneath the image; it repeats the subject in visible, indexable text.
• Put the image near the heading it illustrates; engines read the relationship between heading and image.
• For a product or service, one good image with a caption under the right heading outweighs dozens of generic shots.
• Keep descriptive title off the img (it is tooltip-only and usually noise).

For responsive images, the same image with srcset and sizes gives the engine one URL to index while mobile visitors get a small file. See responsive images with srcset. The exact set of attributes does not boost rank, but consistent markup avoids duplicate URLs for tiny crops.

Image sitemaps

For large image sets or hard-to-crawl galleries, an image sitemap tells the engine directly which images exist and where their landing pages are. It works alongside on-page text, not instead of it. Put the pattern in place before launch and keep it updated when the gallery changes.

Prevention

Image SEO is mostly hygiene, so prevent the costly fixes: standardise filename naming in the CMS, enforce a required alt field, and treat the caption as part of the content. A monthly crawl report that lists images without alt is a reasonable check. For image discovery that depends on the page being crawlable, keep the gallery pages indexable rather than folding everything behind a scripted load.