The house's shadowing. Or maybe it's just me... seeing wrong stuff.
It looks more like Game Maker refusing to do 8-bit alpha on an alpha'd sprite.
anyhow, off to spoiler tags
Well, the problem with gamemaker lite is that you can't use 8-bit alpha channels.
When you use 8-bit alpha channels in GM-pro, you first have a picture, then you have a masking version(like, grayscale). Then you make a little script that uses the masking version to add the varying levels of transparancy to the picture.
While I can type in the same programming in GM-lite, it refuses to work. Because I did want some shading, I made a tile-set version for the grass(hence why is looks so strange).
Now maybe you meant something completely different, in which case I don't know what you did mean...