Are you going to shade the cloud? I think the penumbra on the cactus is maybe a bit too big. The dithered gradient in the sky needs to be spread out more I would've thought, with the sky being darker at the top than it is now. The antialiasing on the cactus is a bit sloppy. Also, I'd up the contrast by making the shadows in this darker, especially in places like the spray can where it seems to have no shadow.
