When I set a wallpaper from the add-ons it has disappeared from the menu of my wallpapers and the start page has defaulted back to this fugly brightly colored graphic design of loud obnoxious jagged stripes. I won't call it painful but I am very sensitive to color :-P and it is annoying I have to return to resetting my wallpaper. I am using the most recent version/update of Opera.
Couldn't reproduce it here, the wallpaper that I've chosen is kept between sessions.
Well on a hunch I deleted a couple of my saved (ones that don't vanish between sessions) already installed from add-ons. I can now keep my moon jelly start page which I enjoy looking at right now because it so very calming. Now I wonder if I have a different bug. Other wallpapers I had installed but thought I had lost (vanished) are now showing. Why would the wallpaper selection not show all I have installed?