For 19.04, we should aim to have a more intuitive wallpaper selection menu. From there, we should have something like a wallpaper contest for people to submit wallpapers.
Well, I think it should happen right after the 18.10 release.
The problem we had in Kubuntu is that we need to guarantee that the person submitting the work actually owns it. Maybe we can talk to Xubuntu and ask how they did it.
But in general, we also need some LXQt refinement to make this wallpaper selection much more friendly. I'll go ahead and put that on the 19.04 list.