Hello Pixelation

I was looking for some feedback on some tiles I'm doing for a game called Super Obama Story
So far I've heard that it could use more contrast and I've given it slightly more contrast, and I believe they may have some minor tiling issues.

the tileset (the bottom right is an old tileset which i used for reference)

an example of them used in a level



I am also looking for suggestions on making further (back layer) tiles for the game

Any suggestions or critiques are appreciated