I've just found and downloaded this free S/W and created a couple of pictures really quickly and easily. Absoultely brilliant!
Here is a description:-
A free project to create digital art using images and computer software. With AndreaMosaic you can create your own photographic mosaics made with your own pictures. A photo mosaic is an image composed of many tiled photos.http://www.andreaplanet.com/andreamosaic/