if you want the Ubuntu wallpapers, install Synaptic (if not already installed) and search "wallpaper", tick all the boxes for wallpaper (be careful not to tick artwork), there are plenty of wallpapers you can install directly!.
Use the following terminal command (Ctrl + Alt + t) or "Welcome" to install Synaptic:
sudo apt-get install synaptic
You can also search the web for sites like the following one!: