Its not an uncommon event for people to steal images of photo sharing sites and pass them off as their own work. Usually it is just a kid who enjoys getting the adulation from their piers on the site. They are usually caught out by other photographer who notice a sudden improvement in the quality of their work or just recognise other peoples shots.
Whilst annoying for the photographer who actually took the shots, the disruption doesn't last long as most sites have staff and moderators who are well experienced in dealing with this sort of thing. But today Paul Indigo brings us news of A stolen picture winning a prize in a photo competition. The brazen cheek of the thief involved is amazing - how did he think he would not go undiscovered.
It begs the question how do you prove that you took a particular image? All I can recommend is that you keep hold of your raw files and make sure they are backed up and archived so that you can get your proof if you need it.
Stephen Baker, the same member of the Fuji website has stolen yet another photo from Trekearth and used it in the current Fuji competition. The photo belongs to TrekEarth member: Antonio (aznegrao) and the photo is named 'The Capoeira'.
Stephen Baker has posted the photo on the competition entry page as seen on the Fuji website.