Well, I suppose, in theory, Google could use it for image searches? Would not make a lot of sense though to index image file references wihout getting the context though... So I doubt Google uses it, but I am not 100% sure right now... Because I need to sleep
Just use the image url as you use a page url. I have no idea whether its useful or not. <loc>http://yourdomain.com/images/yourimage.jpg</loc> Code (markup):
When you use a software to crawl and make sitemap out of your website, you do have the option to include the .jpg and .gif files as well. Yes, these files are submitted to the sitemap, but will not appear in normal search for "pages", but only for "images".