Utility that sets background independently for each monitor

 HydraPaper is a GTK-based utility that sets background images for multiple
 monitor desktops independently.
 .
 It works by merging multiple wallpaper images into one and setting the result
 as wallpaper with the "Spanned" option.
 .
 It is based on the GNOME 3 Desktop but officially supports MATE and Budgie
 as well, and should work on any GNOME-based desktop like Pantheon.