Anyhow, if you're a bit new to Ubuntu, then I'm pretty sure by now that you've figure out that unlike with many other desktops, once inside Nautilus file manager, you just can't right click on an image and "set it as a wallpaper". What!, yep :).
First we gotta open it and then via the default image handler and then by clicking on its window we'll be able to do it. But it's a bit of a work and not the most easiest nor the enjoyable thing to do either.
Luckily, there's a Nautilus extension/script that once installed, lets you easily set any image extension as a wallpaper, it's called "nautilus-wallpaper" (obviously :D).
|Just right-click on any image and from the menu, you'll see a new one called "Set as Wallpaper"...|
sudo apt-get install nautilus-wallpaperBut for the changes to apply properly, you either have to re-login into the desktop or simply restart Nautilus by using the below command.