The file name can give Google clues about the subject matter of the image. The more information the Search Engine crawlers find on your page, the better your ranking results will be.
Try to make your file name a good description of the subject matter of the image. For example, my-latest-website-design-project.jpg is a lot more informative than IMG000123.JPG.
Descriptive file names can also be useful to users: If GOOGLE is unable to find suitable text in the page on which the 'Bots' find the image, GOOGLE will use the file name as the image's snippet in the search results.