The only issue I have with the monsters is how dark their shading is. There's lots of details there and they all kind of blend in together with the dark colours. Nothing a little lightening with colours couldn't fix though.
Not to nitpick the sprites all look great, and I like the direction you're going with the real images as a background, but it seems a little out of sync with the sprites. Sort of as if they were in front of a green screen, they don't seem to connect with the imagery