I've had that happen to me pretty consistently, albeit at a low level, ever since I first started posting art, despite not being prolific or popular. Probably because I allow google searches of my website.

Like, I've had my second comic fanart ever reposted on another LJ and mocked for its slashy subjects, had my HP art reposted on a HP fan message board, found out from someone on my flist that someone else had used one of my slashy Batman sketches for some weird contest thing on a Warren Ellis' site (that person pasted my art into something else, and also removed my sig, so maybe that was "transformative")...

But I fear if Tumblr gets more and more popular and image "reblogging" becomes the norm it will happen all the time.