you can save any image as a .BMP file for use as wallpaper in windows with most image view/edit programs. ACDsee comes to mind, you can load the image and right click it has a set wallpaper option does it all for ya.
why not make your own wallpaper with your fav. pics of the bike. I did it and it looks great. if not look at www.motocrossactionmag.com/picgal.asp they have some pics you can use as wallpaper
Theres A guy on this site with some crazy pictures I dont no if any of them are of an RM125 but there very good. I belive his name is RockChucker but not 100% on that. Check out the galery.