After thinking about this, I then remembered that the game has 2 item.bmp files. One in Graphics.xcr and the other in Effects1.xcr Originally, I thought one was redundant and had no purpose since I edited one and it didn't do anything. I'm pretty sure I edited the one from Graphics.xcr and then looked at the items in my campaign stash.
I now believe that they are both used, but in a different way to the other. One is used in game but the other is used for the campaign stash. Needless to say, you'll need to edit both, if I'm correct on this.