Hey noaudio,
Go to "Downloads" then "Tools" and choose the "
S1res - Firmware changer". Download that and that's what you use to open the firmware file and import or export the .bmp image files and change them to your liking.
I would suggest though that you make a backup of your existing firmware using the
S1fwx - Firmware extractor in case something happens and you need to go back to the original firmware. Make sure you read the instructions on how to use the S1fwx
here.
Make sure you read the instructions and understand what to do before modifying your firmware and updating it yourself.