Search engine bots cannot read the text built into an image, but they can read the alt-text associated with the image and the file name of the image. Both of these can add target keyword content to the page and give you an opportunity to be found in the image search databases of engines. Google Image Search is the fifth largest search engine so ensuring your images are information rich is a solid strategy. For file names of your images, select a system for naming them that fits your development platform AND contains good keywords. It might take some planning up front, but can pay off with big search numbers from the image databases.