They are usally called photomosaics, but robert silvers who does all the really famous ones owns the trademark on hat,
youll find a bunch on goole if you search for the photomosaic
for instance
http://www.fishsoft.co.uk/mosaic_magic.shtml
It takes a lot of source images to make a realy good one, i wrote my own software to do it, and it uses a library of 500,000 images, but for more error margin or a higher repetition and smaller images, you can get away with a lot less.