Someone, usually from Hong Kong it seems, gets it off google image search, then sells it. That's how, in a nutshell. ha. People make thousands of dollars off every possible peice of fanart created, it's ridiculous. WB probably doesn't care enough to get them pulled.
That's not even the best version of that poster
either. And they're printing from a 1000px wide image, which couldn't look that great blown up.