Well... I don't use Gimp. I use Paint Shop Pro ^^. Anyway you'll need an advanced image editor.
You have to extract the image you want to use for the icon from the char, then cut it to 25X25 pixels, save it in a .pcx file, and replace the old one with the new one. (if you don't understand this (most probably you WON'T understand it), I suggest looking for a tutorial vid in youtube).
