The name of an image, along with the "alt" attribute, tells the search engine what the image is about. If a visitor is unable to access the images on your site the "alt" attribute will display text instead. A good file name is also a good reminder to you of what the image is, especially if there are multiple images on a page.
Use brief descriptive filenames and alt text - The filenames and alt text are best when they are short and descriptive of the image.
Supply alt text when using images as links - If you make an image into a link, filling out the alt text helps search engines to understand what it is that they are linking to.
Use commonly supported filetypes - JPEG, GIF, PNG, AND BMP, are the most common image formats.