Patch your ipacsum program if you want transparent images by using the file
ipacsum.transparent.diff. If you do not know how to pach a file, here's a
short introduction:
Most likely you will have to do this as root or you work with you own copy of
ipacsum (set correct path in config.php!)
Fire up some xterm or konsole
# su (change to root)
# cd $(dirname $(which ipacsum)) (go to directory of ipacsum)
# cp ipacsum ipacsum.bak (make backup copy)
# patch ipacsum < /path/to/ipacsum.transparent.diff (apply patch)