i've never tried to make a GUI, but if i were to guess...
- grab an image viewer (id probably use gadget's old one for ease of use)
- export the images from Prguse.wil and Prguse2.wil (think thats the correct file names)
- edit them in an appropriate graphics software package, save as 256 color bitmap
- import the editted pictures back into the 2 .wil files in the correct slots
think thats about it, but ive never done it :P