Just go with it. You can use IrfanView to convert to targa when you're done editing (or if you're photoshop, you can do it directly)The size of the file will change if you delete or insert data, but not if you overwrite, which is what you'll want to do. I know the hex editing program HxD will warn you if you're about to change the file size, but I don't know what program you're using.
Orly? Ok, thanks. Nice to know it's at least possible ahead of time. (Light at the end of the tunnel.)
Edit: Btw, Dolphin dumped me .PNGs. But GodFed wrote in one of his texture hacking posts, here, that it dumps targa files (.tga). Is there a setting that I'm not finding for this somewhere?