Recently, from one of the photography magazines I've read, I discovered AndreaMosaic and the best thing about it is it's absolutely FREE for download. I wouldn't go into details on how to use it because I'm sure you can easily figure out how it works and how to use it. The download comes with instructions in PDF format. Below are samples of some mosaics I have done using the software. Obviously, the more tiles (small pictures) you use, the closer your mosaic will appear compared to the base image. However, I must tell you that after you click the "Make Mosaic" button, you may want to make your self a cup of coffee because it will take from anything between 10 to 60 minutes before it can finish your mosaic.
You can download your copy of ANDREAMOSAIC for free at http://www.andreaplanet.com