GWIS
From ThorxWiki
GNOME's Wallpaper Intelligently Set
This is the script that I use to set my GNOME wallpaper, after I became dissatisfied with the automatic changers that were available.
This script is cronnable, and attempts to intelligently handle images that are bigger, smaller, taller and wider than the display. By looking at relative sizes, aspect ratios, and edge colours, it will set the image to the appropriate type, and specify the background colours and gradient to match.