Every image on your site is an SEO opportunity you miss if you do not attach alt text to it. Google cannot see your photographs. It reads their textual description to understand what they show. Well-written alt text is an extra chance to appear in Google Images and to reinforce your page's relevance.
Why Google reads your images rather than seeing them
Google's algorithms analyse your page's HTML line by line. When they meet an img tag, they read its alt attribute to understand what it contains. The official Google documentation for developers explicitly states that alternative text is one of the main signals used to index and rank images. Missing or empty alt text means an image is invisible to Google. And on an average site, images represent 30 to 50% of the visual content of each page.
The double advantage: SEO and accessibility
Alt text fills two roles at once. It helps Google index your images, and it lets screen readers describe your visuals to users with low vision. Those two benefits reinforce one another: an accessible site sends positive signals to Google, and good image SEO attracts more qualified traffic.
Recommendations on image accessibility are set out in the guides from WebAIM on alternative text: every image needs relevant alternative text, except purely decorative images, which get empty alt text (alt=""). The Moz guide to image optimisation confirms that well-written alt text contributes directly to how pages rank on the associated queries, not only within Google Images.
Basic rule: write your alt text as if you were describing the image to someone over the phone. Be precise without overloading. 125 characters at most, one descriptive sentence, with your keyword if it sits naturally in the phrasing.
The 5 rules for writing effective alt text
- Describe what the image really shows, not what you wish it showed
- Include your main keyword if it sits naturally in the description, never forced
- Avoid starting with 'Image of' or 'Photo of': Google already knows
- For decorative images (dividers, backgrounds), use alt="" so they are ignored
- Give each image unique alt text: no copying and pasting across several visuals
Trap to avoid: stuffing your alt text with keywords. Alt text such as 'website design web agency paris brochure site e-commerce' does not fool Google and can earn you a keyword stuffing penalty. A natural one-sentence description is worth far more.
Auditing your existing alt text
Before fixing, start by measuring. To see in seconds how many images on your site have no alt text, Screaming Frog crawls your whole site and lists every image with its alt text (or the absence of it). The free version analyses up to 500 URLs. For a quick overview without software, Google Search Console flags accessibility and indexing problems in its pages report. The web.dev guide to images additionally sets out best practice on format (WebP, AVIF) and compression, so you can combine relevant alt text with maximum performance.
Measuring the impact on your rankings
Once you have fixed your alt text, follow how your impressions in Google Images move, directly from Google Search Console, filtering the search type on 'Image'. To compare your performance with the competition and identify the keywords your images could climb on, Ahrefs and Semrush publish detailed analysis on the impact of alt attributes on a page's overall organic traffic, not only on image traffic.
Where to start today
Open your browser's developer tools (F12), then the Elements tab. Search for 'img' and check that each image has a descriptive alt attribute. Start with your homepage. The first 5 images are the ones Google crawls first. Fixing their alt text takes under 15 minutes and can trigger new impressions in Google Images at the crawler's next visit.
Alt text is one of the technical fundamentals of SEO, but it will not replace an overall strategy. To lay the right foundations and understand every lever to pull, read the basics of SEO for freelancers and make sure each technical optimisation reinforces your overall position.