Someone reposted an edit I did for Mass Effect. You can argue until the cows come home whether the picture belongs to me or not, but they still reposted work I did and didn't even give me credit for the fact I had to take a picture of the scene, save it, edit it in photoshop, crop it to tumblr specs, then upload it.
And my watermark is pretty obvious so it's not like I just think it's mine that someone uploaded. It is actually mine and you can see the original a few posts down from the reposted one in the tags.
On the plus side (?) generally when you have such art reposted it means you finally 'made it' in the editing world for someone to steal your work like that.
Eh.