I use Lubuntu 15.10 (upgraded from 15.04).
Because some reason, I don't have the wallpaper, or background, folders to the desktop, now. There is no any folder with that names on my PC.
How can I recover it?
It is intended to change the "new" ugly Lubuntu default wallpaper (something like a crystal edges mosaic, or a broken glass).
If it isn't possible... How can I change it?
From where can I get free wallpapers to Lubuntu?
If you want to add back the defualt wallpapers from 15.04 or a previous lubuntu release you can install the lubuntu artwork 15-04 wallpapers Then set this wallpaper set back an acceptable wallpaper from this. OR you could even choose other wallpapers of a previous flavor or release. You can select prefrences desktop settings and choose the file in the filesystem. If you do not know where this is stored you can run dpkg -L in a termnial to list the files and look for something ending in .png.