Don't beat the shit out of yourself, man -- we all start somewhere. It's definitely a good start. My main suggestion would be to work a little harder at matching the lighting between your figures and your background. And don't be too concerned with doing everything, background and all, in Poser. I rarely do everything in one shot -- I like being able to layer things in Photoshop to get exactly the right look rather than trying to fix up a single render.