Image SEO: File Names, Alt Text, Format and Size
How to set up file names, alt text, format choice and sizing so an image can be found on Google Images and does not slow the page down.
rabbitclip teamPublished: 4 min read
Short answer
Image SEO means getting five separate settings right so an image can be found on Google Images and does not slow a page down: file name, alt text, file size, format and sizing. Google Search Central's guidance notes that images are reliably found when served through a standard img element or a picture element; images added purely as a CSS background do not get indexed.
On most sites, image file names stay whatever a camera assigned at random, and alt text is either blank or a sentence stuffed with keywords. Both are wrong; the right approach sits somewhere in between.
Why does the file name matter?
A file name is the first signal Google gets about what an image shows. 'IMG_4821.jpg' carries no information, while 'navy-winter-workwear-coat.jpg' states plainly what the image is.
The rule is simple: lowercase, hyphens between words, plain ASCII characters instead of accented ones. The same image used on several pages can keep the same file name; the real difference gets made in the alt text.
How should alt text be written?
Alt text is the description that stands in for an image when it fails to load or when a screen reader is in use. Google states that the alt attribute is used to describe an image, and that this matters for both accessibility and search.
Good alt text tells someone who cannot see the image what it shows, for instance 'navy winter workwear coat, front view'. Describing the image honestly works better than forcing a keyword into it; leaving it blank costs both accessibility and any chance of surfacing in image search.
- Good: red leather sofa, in a living room, side view.
- Weak: sofa.
- Overdone: sofa couch living room furniture home decor sofa styles.
Which format to choose: JPEG, PNG, WebP, AVIF?
web.dev's material on image formats explains that each format trades off compression differently, and that modern formats such as WebP and AVIF usually hold the same visual quality at a smaller file size. Photographic images generally suit WebP or AVIF, logos and icons needing transparency suit PNG, and simple graphics suit SVG.
Choosing a format is not enough on its own; serving the right format per browser automatically, through a picture element, is how both old and new browsers get supported at the same time. On an e-commerce site, that choice directly affects how quickly product images load.
Why do sizing and file weight decide speed?
Loading an image at 4000 pixels wide when it will only ever display at 400 pixels forces the browser to download and shrink far more data than it needs. That delays how quickly the page's main content becomes visible.
Responsive images solve this by preparing several sizes of the same image and letting the browser pick the right one for the screen. On a spa chain's site, resizing branch photos this way visibly cut mobile load times; no exact percentage was measured, but the direction was clear.
What does an image sitemap do?
An image sitemap is a file that separately tells Google the addresses of images on a site. Google Search Central notes that image URLs in an image sitemap do not have to sit on the same domain as the site itself, so images hosted on a CDN can be reported this way too.
This step is not required for every site, but it speeds up indexing for an e-commerce site that hosts its images on a separate domain or a CDN.
Five steps before uploading a new image
A fixed sequence to follow before any image goes onto a site.
- Crop the image to the largest size it will actually display at; do not upload a camera's original dimensions.
- Save it as WebP or AVIF, and reserve PNG for cases that genuinely need transparency.
- Rename the file in lowercase with hyphens, describing what the image actually shows.
- Write alt text that honestly describes the image, without forcing a keyword into it.
- Once the page is live, check in browser developer tools that the image loaded at the correct size.
Most of image SEO is not design, it is discipline: a real file name, honest alt text, the right format and the right size. Applied together, these five improve both the chance of appearing in Google Images and the page's actual speed. A short discovery call with rabbitclip is enough to see where a site's image setup stands against these five points.
FAQ
What happens if alt text is left blank?
The image loses most of its chance of appearing in image search results, and visitors using a screen reader cannot tell what it shows.
Does WebP work in every browser?
It works in the large majority of current browsers; serving a fallback format such as JPEG or PNG through a picture element covers older browsers without any issue.
Can accented characters be used in a file name?
They should not be; characters outside plain ASCII can cause problems on some servers and browsers, so plain letters are the safer choice.
Does every site need an image sitemap?
No; sites hosting images on their main domain usually do not need one, it is more useful for sites using a separate domain or a CDN.
