pit2_background = base
1/2/3/4 	= version
- 		= divisor
32/64 		= pixel count
5 in the end 	= version.5

How can I change the background?

go to 		config > custombackgrounds.cfg
change     	S:Fixed-Background=pit2_background:pit2_background4-32.png
		"pit2_background4-32.png" to the version you want
