1. Open your ObjectDock settings via the dock icon. See first image.
2. Select the dock you want to change. See second image.
3. Select "Style/Color", then "Change Background." See third image.
4. Select "Open backgrounds folder." See fourth image.
5. Copy your downloaded background into this backgrounds folder.
6. Close the ObjectDock settings window.
7. Repeat steps 1-3 and look for your new background to be listed.