How to change file in ROM

Jonas
Hi everybody, I'm looking now for a while, how to do the simple thing..

I have the car radio, with WinCE 5.0 OS, and I've done some skinning. And
the only one problem what I wanna do, is change the splash screen, which
appears everytime the radio boots up... I found that image, it is a bmp
stored somewhere in Windows forder I thing.. But if I view it in the file
manager, I see the option "in ROM" chacked... So I can rewrite it... 

Actually I can, but nothing happens.
Could somebody here tell me how to rewrite that file, if it is possible?
Thanks.