Your question is a conundrum unto itself.
If you know what a prerendered background is, then you can just render it any format with exception to gif I believe. Stay away from jpeg, as the compression for jpeg while good for photo's on webpages, is crappy for highly detailed backgrounds. You'll tend to get alot of haloing and artifacting when using jpegs. Use png's if you want to easily preserve some kind of alpha channel.
If you want to prerender a background then you have to be alot more specific as to the backgrounds you want to create. What environments, objects, structures need to be in these prerenders? Can't really answer your question specifically since you were so vague.
If you want to render an organic environment like forests and so on, then use something such as Bryce 5+. Bryce is pretty fantastic for organic scene renders. It's not free, but it's dirt cheep at 59-110$ US. Depending on whether or not you want 5 or 5.5 via direct download or boxed. The boxed versions cost 10$ more than the download versions. I'd say pay the extra 10$ for the media and packaging for the boxed version.
http://www.daz3d.com/program/bryce/55index.php
