Definition: An alt tag, also known as "alt attribute" and "alt description," is an HTML attribute applied to image tags to provide a text alternative for search engines. Applying images to alt tags such as product photos can positively impact an ecommerce store's search engine rankings.
Search engines and other robots cannot interpret images, but images can play a crucial part in how people interpret a particular web page. Alt tags solve for this by providing text which is read by the search engines. When Googlebot or other search engine crawlers inspect a page, images with properly formatted alt text contribute to how the page is indexed and where it ranks.
Alt tags are also useful for users viewing a webpage on screen readers or browsers that can't process images.
Alt text is contained within the image tag: <img src="myimage.png" alt="nike_air_zoom" />.
Product images should be optimized for site speed and use the same keyword as the product page itself.
The text should be kept short to maximize its impact. Shorter alt texts are also more likely to be indexed by Google and the other major online search engines in a more efficient manner. Also, do not forget to mention the topic within the alt text.
Refrain from keyword stuffing, as Google and other popular search engines disfavor such antics.
Don't prioritize alt tags over SEO deliverables such as titles, inter-linking, and meta descriptions. Alt tags should only be prioritized for image-heavy pages that are very light on text.