Alternate Image Text or Alt Text is descriptive text that is displayed in place of an image if it can't be loaded, as well as a label on an image when it is moused over in the browser, to give more information to the visitor. Additionally, Search Engines use provided Alt Text to better understand the content of an image. Image SEO is not widely known, but having your image rank for image searches is an overlooked way of gaining traffic and backlinks to your site.
We recommend adding useful and keyword rich Alt Text for pages's main images, in particular those that could have ranking potential. This should be considered on a case-by-case basis. Often there may be imagery such as UI components or tracking pixels where it may not be useful to add Alt Text, though we have tried to filter a number of these out in our analysis.