Since posting his first micro-mosaic, Arthur Gugick (torgugick) has been busy making more. If you'll recall, this technique uses printed tiles, but for a 1x2 tile, half is covered by the next tile. It's done in a stair-step fashion, and allows for a much greater resolution of an image in a smaller space.
Here are a few more of his micro-mosaics, as shown at BrickWorld Indy:
Here is a close-up of the Abe Lincoln micro-mosaic:
In this image, you can see only half of the 1x2 tiles used in the image, and you can also see how the tiles are oriented in different directions to achieve a slightly different quality of color.
Here's one more micro-mosaic, a close-up of Bugs Bunny, just for fun: