Size a picture that has what you want to fit the background and import it into poser (File|import|Background picture) in your render settings (for P5) make sure you have render over background selected. That's the easy way. Another way is to do so inthe material room (in p5). If the background doesn't change, read the SR4 & 4.1 readme for instructions on how to fix it.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)