Thank you Johngalt, seriously.
I admit, I used XP for at least 6 years and never did work with Vista, so I was unaware file owner played a huge part in file modification. Thanks to that tip and some googling, looks like my start button editing can work now.
EDIT: It looks like even after editing all possible Bitmaps, the startbutton has not changed a bit, so this method is rather pointless. That or Explorer is being automatically reverted after I change it, also possible thanks to the automatic system file repairing Windows did (I was aware of this in the past).
EDIT2: I updated the file through XP after taking ownership, and it still did nothing, so I'm wondering if XN Resource Editor can't handle these files yet. Even after such progress of replacing it, it doesn't do anything, so I'm about a step away from not caring anymore.