Excellent guide. Thanks very much for sharing your research. I would only add that you can change the BIFs as well as the TGAs by resizing the TGAs to 18 pixels by 20 in Photoshop, pasting your desired image over, and resaving under a desired filename. You can use CA's BIF/BUF utilities to convert the TGA into a BIF. Then as long as you give it the right name your battle should use it.
The only problem that I still have is that I cannot make the Historical Battles use units from mods, and that I cannot make my units deploy even though I set the deployement (sic) line to "TRUE". Any ideas anyone?
Thanks once again to DragonCrusader.