/background -[amsgdluhcfnrtpx] [window] [filename]
a = active window (window name not required)
m = main mIRC window
s = status window
g = finger window
d = dedicated query window
c = center
f = fill
n = normal
r = stretch
t = tile
p = photo
l = toolbar
u = toolbar buttons
h = switchbar
x = no background picture
:D