How do they make photomosaics - or large picture made of small pictures ?

http://www.photomosaic.com/

How do they make those photomosiacs ? Its almost like the next stage of ASCII art.

Are there any plugins to do this - like Gimp ??

Not a good answer follows:

I know that a few years ago I got a flyer in some software I bought - the company also made software that would make these photo mosaics from your own pictures. Due to my bad memory, I can’t remember the company that made the software - maybe the makers of MGI Photosuite?