This file contains a list of all backgrounds in SMTC right now.  You can use SetBG(index) to swap to a background.  Each BG has its index shown next to it.

Note that these indexes are subject to change.  Check this document at each new release to see if the background order has changed.  Notes will be in the changelog to this effect.

0:  Brinstar Blue (Tiled)
1:  Brinstar Life 1
18: Norfair Bubbles
19: Old Mother Brain