Automatically change backgrounds in Xfce 4.10

1. download a list of background pictures
2. right click on the desktop, and choose "Desktop Settings..."
3. under "Background" tab, choose "Image list" on the right
4. click "+" to add background pictures to the list
5. choose "Change the background (in minutes)", and adjust the minutes
6. if you have one monitor, choose "Stretched" for "Style"; if you have two monitors, choose "Tiled", and choose "Stretch this background across all monitors"