yea hit's called mosaic, and all it really is doing is using the overall tone you get from a small image and using that as a value for a square in a larger composition, but like prodigy91 said you could achieve this effect by overlaying the large image over a bunch of smaller ones and using some blending techniques, might not look quite as authentic but unless you want to go to the process of creating the pattern of light and dark by hand that would be your best bet.