I recently had an altercation with a local entertainment magazine which stole an image right off my website and ran it without contacting me or crediting me. Needless to say, when I found out a letter went right to the magazine owner with a bill, which he balked at, and proceeded to berate me over the phone. Pissed, I made a few inquiries to friends and one suggested I contact a copyright attorney who does a lot of free consulting for artists and musicians. After a 15 minute conversation with the guy, I had all the tools I needed to go back and confront the paper owner and we settled on a price and they ran a correction.
Two of the things I came out of my convo with the lawyer, 1., no matter how trivial, it pays to get your images registered with the US Copyright office, it's easy, cheap, and you can do it on-line and 2., you don't ever want to go to copyright court with a person or corporation who has more money than god, they will destroy you with lawyer fees, delays, and basically ruin you financially and even if you win, they may appeal til the end of time. Better to try and settle the case.
I don't know if this has anything to do with the discussion above, but yes, the studios own copyright on all the poster images, they probably just don't have enough time to go after such small potatoes such as people using the images on websites.