You have way too much going on in all of these
Try not to make everything transparent
Like idle said, you need to add shadows to make things look like they are actually there. Not sure if gimp has a burn tool, but that is what you can use. Or just paint with 5% black.
Just keep your file at 400*150 and zoom in if you can't see.
This post was edited on 12/28 at 2:51 pm