I honestly couldn't tell you. Because, I have
had themes install wallpaper right into the iPad's stock Wallpaper file. For example, the inPulse for iPad
theme put backgrounds into that folder (accessible via Settings > Brightness & Wallpaper > Wallpaper). I don't use them, but they are easily got at.
In addition, I've installed other themes that do put backgrounds on as part of the theme (automatically). I think that's why the "User Backgound" section was developed, so users didn't have to use wallpaper that loaded automatically as part of a theme.
All that to say, who knows why this guy didn't/couldn't put the wallpaper into the theme. All I'd suggest is re-trying those other themes and then see what is going on with them. All the developers/themers have their own methods - we just have to figure them out.
Anyway. Glad we got it sorted.