How the Search Engines Handle ImagesPittman says its no longer just about looking at tags and stuff, at least at Google.

We now use computer vision and search to find similar images, object recognition, and facial recognition, he says. We look at everything such as exposure data to help determine the quality of an image. If your image quality is low your photos will not rank as high. If you make only a thumbnail of an image it wont rank as well as a larger image of the same subject.