The best way I have found to add backgrounds is->
1. find the file/picture on your USB drive, right click and then select "copy"
2. Go to your home folder, or pictures folder and right click then select "paste"
3. Close the file manager window
4. Right click desktop. select desktop settings.
5. At the lower left bottom under the current backgrounds it has a "Folder" selectable button. Select "other"
6. Navigate to your home or pictures folder where you put the file/picture. You can select the ENTIRE folder as well.
You should now have the picture you desire as your background.
To add pictures to the normal location they are located in-
1. open file manager and go to- "File Sytem" drive on left pane-->and navigate to /usr/share/backgrounds/xfce
2.open the folder and right click in a blank spot. Select "Open as Administrator"
Enter password.
3. the folder now opens with read write access.
4. Add your desired pictures to the folder.
5. For each picture added, right click and select "properties", under the "permissions" tab there is "others", set it to "read"
close the box.
They should now be selectable in the backgrounds selections in the default location.
