You can use any sprite group, just edit the following section in your system.def:
;Victory screen background
[VictoryBGdef]
[VictoryBG 1]
type = normal
spriteno = 100,0
start = 0,0
tile = 1,1
velocity = 0,1.5
window = 0,501, 1279,959
[VictoryBG 2]
type = normal
spriteno = 100,1
start = 0,0
tile = 1,1
window = 0,0, 1279,500
velocity = 0,1.5
[VictoryBG 3]
type = normal
spriteno = 100,1
start = 0,0
tile = 1,1
window = 25,520, 1254,709
velocity = 0,1.5
I copypaste'd this from the mugen1 screenpack (1.0's hi-def default), as you can see it uses the 100 group in case you want to do a simple edit and not a complete revamp.
