1.) Download the Halo 2 Bitmap Internilizer by TfAv1228 -
http://files.halomods.com/download.php?id=44363l
2.) Run the program, open up your map, find the bitmap you are changing, hit the internilize button, then hit the fix encryption button.
and close your map.
3.) Open Dot Halo or what ever you use to inject ur skin, and inject it, sign and ftp.
I think thats all
NOTE: Someone correct me if im wrong but im pretty sure thats right.