Legend is a program which I made when I use was using Windows and Visual Basic. You give to it a picture, let's say a photo of a silly friend, and then a directory full of, for example, porn pictures. :-) Legend will create a photographic mosaic with the porn pictures that resembles the photo of your silly friend. It has an option to enlarge the resulting photo so that you can make it a poster. So print it and send it to your friends mum. ;-) Of course you can find much better uses for this program, like making a picture of a politician out of starving children or perhaps a picture of a flower made out of pictures from your holidays.
You can download Legend by clicking here.
Here is a screenshot of the program: