You can indeed put a custom image on your weapons (on anything really). You just have to ensure that it looks similar to the image of the weapon you are replacing, so no putting a revolver image in for a DL-44 for instance.
Cowboy Bebop? I dont think I seen revolvers in there. I know Spike uses a Jericho 941 which is def not a revolver. Jet Black Used a Walther P99 and Faye Valentine used a Glock 30 and the Kid is never seen with a weapon I can think of.