The first and most apparent problem I see is lots of
pillow shading on the...the...
...Not to be rude or anything, but to tell you the truth I've never seen a dumpster like that. Is this a dumpster, or a log cabin?
First things first, you should pick out a lightsource before you start shading the dumpster. This is a common mistake usually made by the newest members of the pixeling community.
Very quick edit. I might have made it a bit too dark, though.
