Every person seems to have a running visual monologue going in their head as they listen and read. These visual images are reinforced by Hollywood, TV, magazines, books etc. I was doing a presentation for teachers last fall. One of the questions I ask is if I say the words American Indian or Native American: What picture do you see in your mind?
I followed this up by using Montage-a-Google and typing in Native American. We talked about how most of the images were of Native Americans from the past. One of the issues we are working on within our schools is the need to change those perceptions. Creating an awareness that there are different tribes that are a living vibrant contemporary culture. The second mosaic shows contemporary tribal members. We had an interesting discussion about perceptions.
This site would also work to introduce other topics in history and social studies, even biographies. If you click on individual squares it will bring up the original picture so that you can examine it closer, allowing for further discussion.
There is a great article on Engadget : How to Make Your Own Photo Mosaic. They list a couple of other sites that you can use to make your own mosaic from your own pictures: Andrea Mosaic for PCs and MacOSaiX for Macs. You can also make mosaics with Picasa.