Image alt tags help users as well as bots
Image "alt" tags are the descriptive text that has been assigned for a given image. Not only are these crucial for people with visual disabilities who are unable to view, but must instead hear a screeen-reader narrate the contents of the image, but also for search engines. This is because search engines are not as good as humans at describing the content (or intention) of an image as well as humans.
Search engines will therefore reward you two fold if you include alt tags for all your images - so just do it for an easy win!