The mosaic of the kiwi and strawberries up above was made with a collection of images (replicated to only 500 tiles).
Looking for a similar free program for your Mac? MacOSaiX works similar to the PC version mentioned above. Choose the image you want to make a mosaic from....choose the shape and number of tiles in the mosaic....choose the sources of images to use for the tiles....and step back while your mosaic is created.
Have some fun....create your mosaic then create some wall sized posters from the mosaics for your classroom using The Rasterbator or Blockposters.com.
Other Mosaic Links: