Originally Posted by mortenb

One thing I've helped set up for a couple of sites is to add the username of the downloading user to the exif data of the images they are downloading. This won't block them from downloading the images, but at least it will allow you to see which user downloaded and shared them. This method is in no way foolproof, as the exif data can be removed or overridden, but most users don't even know it is there.