If you have the privilege you can upload new backgrounds for everyone to use. The Library | Backgrounds screen gives you a button for this purpose, .
Once you click it, the Upload Background screen appears. This screen allows you to upload up to five (5) background images at once.
The general steps to follow are:
Name the background. Each background name must be unique.
Use your browser to locate the background image file.
Choose an aspect ratio for the background. This "primary" aspect ratio should most closely match the aspect ratio of the image file.
Upload all the images you've selected by clicking .