Depends on the images. For photographs, a digest is enough. For "random images downloaded from the web", or when you're deduplicating lots of user's data, sure,

Depends on the images. For photographs, a digest is enough. For "random images downloaded from the web", or when you're deduplicating lots of user's data, sure, you want data appropriate digests, like SIFT prints. But then we're back to "you had to bring network content back into this" =)