Hopefully this does the trick
Plug your mogul into your computer, explore your device: my device > My Windows Mobile-Based Device > Windows. Make sure you are able to view hidden and system files on your computer. You will find the name of your background image on your Today screen is two files, one for when the keyboard is closed and one for when it is open, they are "stwater_240_320.bmp" and "stwater_320_240.bmp". That is what they are named on mine anyway. I hope this helps.