The program works quite well, and just has one icon. It essentially lets you choose a folder that has your pictures, and lets you choose the timed or random intervals of what your wall paper will be.
http://sourceforge.net/projects/wpchanger/
It is posted on Sourceforge, which I think does that mean it is open source. It has the GNU General Public License (GPL) as it's license.
I am not sure if it is the smallest file size, but it is one that works.