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?