According to engadget a 'photo mosaic is one large image made out of hundreds and thousands of tiny images from your personal stash, from google images or even frames from a movie. There are a few applications that do this for free, on Macs / PCs.
In the field of photographic imaging, a photographic mosaic (also known under the term Photomosaic, a portmanteau of photo and mosaic, trademarked by Runaway Technology, Inc.) is a picture (usually a photograph) that has been divided into (usually equal sized) rectangular sections, each of which is replaced with another photograph of appropriate average color. When viewed at low magnifications, the individual pixels appear as the primary image, while close examination reveals that the image is in fact made up of many hundreds or thousands of smaller images. They are a computer created type of montage.
See the full Wikipedia entry on photomosaic here.