Although Rumblah! Beta 0.51 only let's you choose between 2 background you can still work around it and play in your own Backgrounds.
Let's say you already have a new working background, either downloaded or created by yourself.
- Put the .swf file (and anything extra that you need if it a more complex background) of your background inside the /Bkgs directory in your Rumblah directory (Default path: C:/Program Files/Rumblah/Bkgs). Let's say it's name is MyBkg.swf
- Let's say we want to replace the TrainingRoom for your new background. Inside /Bkgs , right-click TrainingRoom2.swf and select 'rename'. Rename it to 'TrainingRoom2_original.swf' or something along those lines so you know it is the original background.
- Rename 'MyBkg.swf' to 'TrainingRoom2.swf'. If you did everything correctly then when you choose the TrainingRoom in Rumblah! it will load YOUR background and not the original Training Room.

- Enjoy!