Customising the Gnome 3.x Desktop
In Fedora (-and other Linux distros using Gnome 3) the desktop wallpaper can be changed by left-clicking on the User Menu and selecting the "System Settings" option:
In Linux Mint, the "System Settings" option is similar:
A window will open up, listing a number of system functions that you can invoke. Click on the "Background" icon:
In Linux Mint, the option is called "Appearance" rather than "Background" (-hint: it is still the first option listed):
A dialogue box will open, showing a number of pre-installed backgrounds that you can use - just click on any one of these to select it as your background:
If you want to use one of your your own images, you can either click on the "+" icon (-under the list of wallpaper) - or click the down arrow after the "Wallpapers" label and choose "Pictures", which will allow you to navigate to the required file.