Couldn't find the demo, but a couple of other freebies.
http://www.radivarl.demon.co.uk/nick...k/mosanick.htm - the example on the page doesn't look very promising, excessive reliance on "fade to white" - with this kind of program, maybe pre-manipulating some reshaded versions of your original mosaics may be better - you need to give it plenty of ammunition to work with!
http://www.alhena-design.com/centarsia/ - another one, and if the flag on the images page is anything to go by, maybe the best of them
There's also TYLER
http://www.sover.net/~wysiwygx/Graphics2.html
ftp://ftp.externet.hu/pub/mirror/sac/graph/tyler.zip
Tyler Creates a large image using libraries of other images. Last freeware link... get it today even if you're not sure you'll EVER need it!