I don't have experience creating movie poster banners or anything, but I do create my own site graphics and banners. My only advice would be to stop trying to cram all the movie poster information into a skinny banner and just select key elements to represent. Such as the new X-Men Origins banner you see on that site you linked. They just took the main character, stylized him and put a logo on the bottom. If you make it catchy enough, and use it as a link to an information page about the movie, you'll get your point across easily enough.
You may be able to get pre-made graphics from the production company, I'm not entirely sure on that one. I would think that they have promotional packages for movie theaters to help promote their products.
I would be very cautious when "borrowing" another websites graphics. Most sites really do not approve of such things. I would get in contact with the sites admins and ask them for their permission (and make sure they aren't using the images under some underlying license, because then you may need more than their permission).