So, Thomas has this picture in his house, and I just (well, two months ago) realized it was a Banksy.
This link will take you to the page of his rat pictures, the fourth one being the picture in question.
Now, maybe Thomas had told me it was a Banksy and I just forgot, but it was still a surprise to spot it on the Internet. ^_^
Update: Well, I asked Thomas about it, and no, he hadn't told me it was a Banksy, because he was told that it was some California (con-)artist. So some Cali guy is painting up Banksy reprints and selling them as his own. That rat bastard.