How can I change the picture used by this AppleScript?
You can see all the images in that folder by opening a Finder window,  pressing Cmd-Shift-G, and entering the folder name /Library/Desktop Pictures; or by running ls '/Library/Desktop Pictures' in Terminal.

pse@Mithos:~$ ls '/Library/Desktop Pictures'
Abstract/                   Desert 4.jpg                Galaxy.jpg                  Reflection 1.jpg
Abstract 1.jpg              Desert 5.jpg                Grass Blades.jpg            Reflection 2.jpg
Abstract 2.jpg              Desert 6.jpg                Hawaiian Print.jpg          Reflection 3.jpg
Abstract 3.jpg              Desert 7.jpg                High Sierra.jpg             Reflection 4.jpg
...


Choose one of the images and then put its name in the place of Earth Horizon.jpg in your script like this:

osascript -e 'tell application "Finder" to 
    set desktop picture to POSIX file "/Library/Desktop Pictures/FILE_NAME_HERE.jpg"'