I've had it happen before but not in such a large forum of negatively.  The admin of a group on Facebook took one of my photos off a Google search, and used it as their icon or avatar.  Now, it was a minor sort of HDR but it's more the principle of the thing.  Instead of saying "Sorry, I didn't know that content on the Internet has an intrinsic copyright and I'll take it off right now" or asking me permission to use it (which I would have given); he and some of the other sin the group argues with me about whether they have the right to use it.  Despite giving them references to websites about copyright, etc.  

One person even said something to the effect of "I found it on Google, if you don't want people taking your stuff take it off Google."

There's a certain amount of various levels of ignorance there that I find impossible to argue with.  What do you say to someone like that?

Fortunately Facebook has a real nice complaint sequence for making copyright complaints.  We'll see what comes of it but I was able to reference the group's image, where it was located, and where the original work was and the URL to it to confirm it was mine and they didn't have permission.