Yep, canvas frames are what you're looking for, like David said.
Or, move the posters outside the entrance to the theater. I opted for that solution, using a TV as a makeshift electronic poster. Not only can I display nearly any poster ever released, I can also display custom greetings for our guests, like these:

Movie poster dilemma
It's hard to see on the reduced image, but the marquee shows the name of the movie and our guests, which is on a slideshow along with the movie poster and the Now Showing title.
I'd like to upgrade to Samsung's The Frame, but the control box is obnoxious and would look out of place.