1) User uploads a directory of desktop backgrounds. 2) When a specified amount of time passes, the desktop background changes to the next image in the directory. Thanks for considering
Good news, this will be implemented in the next xfce release. It's based on the design SIG: http://wiki.xfce.org/design/xfdesktop/settings You can test in out if your distribution supports some way of checking out unstable/dev releases (i.e. PPA's for ubuntu or overlays in gentoo). Some of the Xubuntu people were kind enough to write updated docs for the next release, you can check them out here: http://smdavis.us/doku/doku.php?id=xfdesktop-docs
Marking this as resolved as it's implemented in git and will be available in the next xfce release.