While wandering around Flickr admiring all the wonderful talent out there, I discovered free software called AndreaMosaic. This software converts your photos to mosaics using the photographs you provide. The only requirement of the licence is that you credit Andrea when you use the software.
To get a good mosaic I think you need to spend a bit of time playing around with it and you need LOTS of photographs and MANY variations of colour. Here is my first one - you will notice that all the tiles are other birds. As a quick trial I only included 50 photographs for the tiles and limited the number of tiles.
This is the example that Andrea gives of the mask file showing that you can draw your own...
I have loads of pictures of flowers so I included 2 directories of flowers that I had taken in gardens and flower shows. I then chose a daisy picture:
and converted it to the necessary maskfile: