1. put them on a usb
2. go to the usb via dashboard
3. click on the picture and use as background
4. it should now be stored with all your dashboard backgrounds in user prefreneces
If that dont work get back at me cos im going off memory at the momment.
i got a load of pics on my xbox360 but i dont think you can transfer them off again, only delete.
edit: you might be able to take them straight off the disc via background settings, but once again im going off memory.