For the motivational posters specifically, you might want to check out a Perceptual Hash type algorithm. Convert the image to a 64px square low-depth grayscale (4b?) and most of them should look more or less the same to PHash. Maybe you then classify them into clusters based on hash distance or something.

I also like your dominant-color thing. If you have a couple approaches that each make sense, you can use them as an ensemble - the more classifiers that don't like something, the more likely it's junk.