IMO, Imagination should be what you see and feel in your own self, and not directed by another. Reading, and daydreaming, are the best ways that I've found myself be the most imaginative. Just turn off the rest of the world and focus on your book (or the vast reality that is in front of you in the case of daydreaming).
Books will encourage your imagination to see what is being described, daydreaming will encourage your imagination to see all realms of possibility. I enjoy both :)
Movies do tend to distort perception. After first seeing a movie with a properly casted character, I have a difficult time seeing the original vision I had of that character in my head. On the flip-side, when a movie gets the character so terribly wrong, I tend to be a terrible critic of the movie/series.