I'm just trying to figure out how to make my own bullet hole image. I'm using a standard Roblox gun, Phantom Forces style, so it works already, but when I change the bullet hole decal ID to my own instead, which is http://www.roblox.com/asset/?id=64291961, instead of the standard http://www.roblox.com/asset/?id=2041947104, the bullet holes stop appearing. I have no idea why the ID would matter.
You're using the decal ID instead of the asset ID, if you paste the decal ID into a decal and then put it into an image or whatever you're using it for it will automatically change it to the asset ID of the image.
2041947096 is your asset ID and in scripts you should change decal ID's with the string "rbxassetid://2041947096"