If you want to just make a background for your Poser scene, one option is [POV-Ray](http://www.povray.org), which I used on the attached picture. I use it a lot to make background pictures that I import into Poser. It's probably a lot harder to use than Bryce, but it does have the advantage of being free. :) It is possible to import Poser scenes into POV, but it's a pretty big headache, so I normally just use POV for backgrounds.

Another possibility for making backgrounds is Paint Shop Pro; it's "Balls & Bubbles" filter can make some interesting images.

Hope that's helpful,
Jack