can't say this is correct as i haven't changed it myself, but i did change the phone and contacts icon using this method and having seen the green android image in teh same apk i can say these instructions are 99.9% surely the steps you would take...
you need to change the phone_default_icon.png file in the phone.apk drawable-480x320 folder, resign the zip and flash from recovery menu...
R
EDIT - you will most definitely need a rooted phone suing my method...