You have probably seen advertisements using the technique of building a picture from miniature versions of other pictures. (Best viewed from a few feet away) There are commercially-available program to do this, but ImageIt! is much cheaper. (One program I located costs over $1000 and is very un-user-friendly, since the creator doesn't have English as a native language.)

ImageIt! comes with a database of 2000 processed pictures (only 1KB each, unzipped), but you can create your own as well by simply selecting a folder and clicking a button. You can add to the existing database or create a new one. The more pictures in the database the better, so that the program will have a better chance of finding images that match the average color of a given square of the input picture.