The wallpapers are stored in one of the folders in the Images section (under Pictures And Video, or whatever precisely it's called). I know how to get rid of them, because I deleted the ones in mine (now I just have a blue screen), but I'm not entirely sure how to add new ones. Probably by putting some on a USB thumb drive and plugging it in. Possibly you might be able to use the TV's web browser and download some images to that folder.