Could the wallpapers be hard-coded as well since you can't get rid of them either?
I'm not entirely sure Hank.
When I first got my 9900, I updated the OS and shrank it in the usual way using BBHybrid Tool, checking the default background images to be removed. The wallpapers were deleted from the phone, and have been gone ever since. I thought nothing more about this until recently.
When I got my 9790, I did exactly the same thing however the wallpapers remain, and I cannot remove them no matter what method I try.
The default wallpaper cod in previous OS versions always used to be something like net_rim_bb_media_backgrounds_640x480b.cod and when this file was removed from the OS, the wallpapers were deleted from the Media folder. I definitely removed this cod from my 9790 OS, but it does look like the background images are locked into the device as I cannot delete them now.