Filename, alt text, format: the three image decisions

Three decisions, made once per image

Filename first. Google's documentation gives the example directly: my-new-black-kitten.jpg is better than IMG00023.JPG. Short but descriptive is the whole rule, and a camera default fails it.

Alt text second, written to be useful and information rich and in context of the page content. That last clause is the part usually missed: alt text is not a place to repeat a keyword, it is a description that has to make sense next to the paragraph it sits in.

Placement third. Images should be near relevant text and on pages relevant to the subject. A gallery of unrelated pictures at the foot of every page fails this quietly.

The technical choices, counted

Format first. Google Search supports images in 7 formats: BMP, GIF, JPEG, PNG, WebP, SVG and AVIF. For photographs on a business site the practical shortlist is 2, WebP and JPEG, and a house rule of capping the long edge at 1920 pixels keeps files small without visible loss.

For responsive images, Google recommends always specifying a fallback URL in the src attribute alongside srcset or picture. That is 1 attribute that prevents an image from disappearing entirely for a client that does not understand the rest.

Two more numbers decide whether the image hurts you. Explicit width and height are 2 attributes that stop the text jumping when the picture arrives, which is what keeps Cumulative Layout Shift under 0.1. And where discovery is a problem, an image sitemap provides URLs Google might not otherwise have found.

The three decisions and the two that follow

Decision	What Google asks for	Cost
Filename	Short but descriptive	Seconds
Alt text	Useful, in context of the page	A minute
Placement	Near relevant text, relevant page	Free
Fallback src with srcset	Always specify one	One attribute
Width and height	Prevents layout shift	Two attributes

Filename, alt text, placement and format list from Google's image SEO documentation.

What to do with an existing site

Do not rename everything. Fix the images on the pages that matter, which is usually under a dozen, and let the rest keep their names until the page is next touched.

Write the alt text last, with the finished paragraph in front of you. Written before the text exists it becomes a label, and a label is what search engines have been discounting for years.

Count the work before you fear it. 12 images on the pages that matter, 3 decisions each, is 36 decisions and about 1 minute per image, so under 20 minutes for the part of image SEO that Google actually documents.

Questions and answers

Does the filename really matter?

Google's documentation names it directly and gives an example of a good and a bad one, so it is at least worth the seconds it costs.

What should alt text contain?

A useful, information rich description that makes sense in context of the page content, not a list of keywords.

Which format should I use?

Google supports seven. For photographs WebP or JPEG covers it, capped at 1920 pixels on the long edge.

Do I need an image sitemap?

Only where images might not otherwise be discovered. It is a discovery aid, not a ranking device.

Sources

Google Search Central, Google Images SEO best practices Filenames, alt text, placement, supported formats and image sitemaps
web.dev, Web Vitals The Cumulative Layout Shift threshold that image dimensions protect