I love studying movies through scene by scene screencaps. Breaking down films to frames both exposes the artificiality and the art of moviemaking. 

These are a few sites I frequent for these:

deep fried movies - no fuss, but limited selection.

caps a holic - gets into the nitty gritty of the source material, often housing screencaps from multiple sources.

scene still - focuses on high resolution images. 

film grab - large library, includes ads but is not too obnoxious about them.