You can, just need to use a bmp, or png >:D
/help /background

/background [-aemsgdluhcfnrtpx] [window] [filename]
Changes the background picture setting for a window. This can also be changed via a windows System Menu.

-a = active window
-m = main mIRC window
-s = status window
-g = finger window
-d = single message window

-e = set as default

-cfnrtp = center, fill, normal, stretch, tile, photo

-l = toolbar
-u = toolbar buttons
-h = switchbar

You can right-click in the toolbar/switchbar to pop up a menu for changing these settings. Toolbar buttons can use RGB Color 255,0,255 for transparency, the BMP must be of the same form as that in mIRC resources. It should be a 16 or 256 color BMP.