Disable Set as Desktop Background altogether - found the solution!

This article explains how to disable the desktop background option:

http://support.microsoft.com/?LCID=1033&kbid=327998

However, in order to disable it, the administrator has actually to set a background wallpaper. Eh? How stupid is this? I don't want
to set any stupid background images. Plus, I wanted to disable this option altogether so as to avoid accidentally clicking this
option in the IE and WinExplorer contextmenus. It is not possible to do this through gpedit (it works for IE, but not for
WinExplorer), though.

But I found the solution. Go to the following keys:

HKEY_CLASSES_ROOT\SystemFileAssociations\.jpeg
HKEY_CLASSES_ROOT\SystemFileAssociations\.jpg
HKEY_CLASSES_ROOT\SystemFileAssociations\.gif
HKEY_CLASSES_ROOT\SystemFileAssociations\.png

and set the value ImageOptionFlags to 1. Now if you rightclick an image in Vista explorer, the "Set as Desktop Background" is gone