For your images to show up in Google Image Search, you don’t just need to make sure your images are on the web somewhere, you need to make sure that the image itself is on a web page that can also be indexed. If either the image itself or the associated page the image is viewable on is not able to be crawled or indexed, then Google won’t show it in image search.

John Mueller from Google offered this advice once again on Christmas day in a Google Webmaster Help thread. John was responding to a complaint about an image in Google Image Search. He said:

Forum discussion at Google Webmaster Help.