Search Engine bots can see and read only texts.They cannot identify images.Hence optimizing the images is very important as far as
SEO is concerned.Images must be described with relevant text that includes your top keywords.This is useful if at times due to some problem if the image could not be loaded ,then the alternate text could tell us what the image describes.
Care should be taken so that the same text should not be used for two or more images on the same page.