Well, I think I found the answer. First, you should take ownership of
/usr/share/backgrounds/contest/precise.xml using the
chown command. Then you can copy your favorite image(s) to
/usr/share/backgrounds. Then you can modify
precise.xml to do your desired job.
precise.xml in a text editor. Suppose the image you want to add is named
ABSTRACT-AnotherBlue_1024x768.png. Then you should copy one part of the body program and modify it like:
Note that the next image name is
Twilight_Frost_by_Phil_Jackson.jpg, so you should copy this piece just before:
Now save and exit. Then restart your computer. You're done!