I am at work so I can't test it but you may have to put a couple of syncs after you paste the image and you may bave to turn the backdrop off. This might work. If not I'll try it when I get home as I have never tried this. I know you can do it with sprites by using draw sprites first command.
set display mode 1024,768,32
sync on
hide mouse
color backdrop rgb(0,0,0)
load image "fishscale_rainbow.jpg",1
load image "swgwallpaper_fett_1280.jpg",2
paste image 2,0,0
sync
sync
make object cube 1,10
texture object 1,1
position camera 0,20,-20
point camera 0,0,0
val#=0.5
do
yrotate object 1,OBJECT ANGLE Y(1)+val#
xrotate object 1,OBJECT ANGLE x(1)+val#
sync
loop
[edit] No that doesn't work. You can do it with a sprite like
set display mode 1024,768,32
sync on
sync rate 0
hide mouse
backdrop off
draw sprites first
load image "fishscale_rainbow.jpg",1
load image "swgwallpaper_fett_1280.jpg",2
sprite 1, 0, 0, 2
set sprite 1, 0, 0
make object cube 1,10
texture object 1,1
position camera 0,20,-20
point camera 0,0,0
val#=0.5
do
yrotate object 1,OBJECT ANGLE Y(1)+val#
xrotate object 1,OBJECT ANGLE x(1)+val#
sync
loop
