The dual purpose of alt text
Alt text (alternative text) was designed for accessibility: it describes images for users who cannot see them, including screen reader users and people with slow connections where images fail to load. Google also uses alt text as a primary signal for understanding image content, making it critical for image SEO. Effective alt text describes the image accurately and concisely, includes relevant keywords naturally, and provides context for screen reader users. The alt attribute should be added to every content image in your HTML. Decorative images that do not convey meaningful content should have empty alt attributes (alt="") so screen readers skip them.
How to write great alt text
Good alt text is specific, concise, and useful. Describe the subject of the image, its key characteristics, and its context on the page. Aim for 8 to 15 words, or under 125 characters, which is the point where some screen readers begin to truncate. Include your target keyword if it fits naturally, but do not force it. For example, alt="vintage brown leather armchair in a living room" is better than alt="chair" or alt="best vintage leather armchair cheap free shipping". If the image contains text, include that text in the alt attribute. If the image is a link, the alt text should describe the link destination rather than just the image content.
Alt text mistakes to avoid
Never leave alt text blank for content images, as this misses both accessibility and SEO opportunities. Do not use alt text that starts with "image of" or "picture of," as screen readers already announce that an image is present. Do not stuff keywords or write alt text that is irrelevant to the image content. Do not copy the page title or meta description as alt text for every image; each image deserves a unique description. For complex images like charts, diagrams, or infographics, provide a longer description in the surrounding text or use the longdesc attribute, as alt text alone may be insufficient to convey the full information.
Relevance to page topic
Google ranks images based on the relevance of the surrounding page content, not just the image itself. An image of a chocolate cake will rank better for "chocolate cake recipe" if it appears on a page about chocolate cake recipes, with relevant headings, body text, and a recipe. Ensure that every image on a page is thematically related to the page topic. Avoid placing irrelevant images just for visual interest, as this can dilute topical relevance. Use descriptive captions near images when appropriate, as captions are read by Google and provide additional context. The image should be placed near the relevant text that discusses its subject.
Image placement and captions
Place important images near the top of the page or near the relevant content section. Google gives more weight to images that appear in the primary content area rather than in sidebars, footers, or below extensive unrelated content. Use descriptive captions when they add value, especially for data visualizations, before-and-after comparisons, and product photos. Captions should be concise and informative, not redundant with alt text. Avoid using generic captions like "Figure 1" that provide no context. Ensure that images are not hidden behind tabs, accordions, or lazy loading that prevents Google from discovering them during crawling.
Unique images vs stock photos
Original, unique images tend to perform better in Google Images than generic stock photos that appear on thousands of other websites. Google can detect duplicate or near-duplicate images and may only show one version in results, typically from the highest-authority domain. If you use stock photos, customize them with overlays, cropping, or composition changes to make them unique. For product pages, use your own photography rather than manufacturer-provided images, which are often duplicated across many retailer sites. Original screenshots, diagrams, infographics, and data visualizations are particularly valuable for image SEO because they are unique and often earn backlinks from other sites citing your data.