Images and Alt Text

Images are the part of a page a search engine cannot read. Everything Google knows about a photo comes from the text you put around it. That text does double duty: it is also how blind visitors experience your images, which makes this one of the few SEO tasks that is straightforwardly worth doing for its own sake.

What alt text actually is

Alt text, short for alternative text, is a written description attached to an image in the HTML:

<img src="sourdough.jpg" alt="Round sourdough loaf with a scored cross on top">

It is not visible on the page when the image loads normally. It is read aloud by screen readers, shown if the image fails to load, and used by Google to work out what the picture shows.

The test for good alt text is simple: if you read it aloud to someone who cannot see the screen, would they understand what is in the picture? "IMG_4471" fails. "bread bread bakery fresh bread Tehran" fails differently. "Round sourdough loaf with a scored cross on top" passes.

One important exception: an image that is purely decorative (a divider, a background flourish, an icon next to text that already says the same thing) should have an empty alt, written as alt="". That tells a screen reader to skip it. Leaving the attribute off entirely is different and worse: the screen reader may then read the filename out instead.

How Google uses images

Google builds its understanding of an image from several signals together, not from alt text alone. The filename matters: sourdough-loaf.jpg tells Google something and DSC00291.jpg tells it nothing. So does the surrounding text: a caption, and the paragraph the image sits in, give Google the context that the picture alone cannot.

There is also a reason to care beyond the page itself. Google Images is its own search surface, and a well-described photo can bring visitors who would never have found the article through ordinary search.

Then there is weight. Images are usually the heaviest thing on a page, which makes them the usual cause of a failing LCP score, the Core Web Vitals measurement of how long the main content takes to appear. An enormous unoptimised photo is simultaneously an accessibility problem, a speed problem and a ranking problem.

What to do

• Write alt text that describes the image, in a normal sentence, for every image that carries meaning.
• Use alt="" for decorative images rather than omitting the attribute.
• Rename files before uploading. Ten seconds each, and it lasts forever.
• Compress every image and use a modern format such as WebP. A photo straight off a phone is often several megabytes and needs to be a fraction of that.
• Always set width and height so the browser reserves the right space and the page does not jump as images arrive.
• Add a visible caption when the image needs explaining. Captions are read by more people than body text.

Common mistakes

Treating alt text as a place to put search terms. Google's own image guidance says to avoid stuffing keywords into alt attributes, and calls it out as spam. It also actively harms the blind visitors the attribute exists for.

Starting with "Image of" or "Picture of". A screen reader already announces that it is an image. Describe the content and skip the preamble.

Uploading at full camera resolution. A 4000-pixel-wide photo displayed at 600 pixels wastes the visitor's data and your speed score, and looks identical.

Sources: Google Images SEO best practices and Understanding page experience.