Windows only: If you're going to create a photo mosaic, you might as well invest the time to do it well. Foto-Mosaik-Edda is a powerful tool for creating impressive, detailed, finely-tuned photo mosaics.
To use Foto-Mosaik-Edda you need a pool of source images—the bigger the better, as they recommend 2000 or more—and the actual image you want to base the mosaic off. In the case of our sample image we used 1,800 images of Marilyn Monroe and found that Foto-Mosaik-Edda did an excellent job using the variety of images we provided.
When you first use Foto-Mosaik-Edda, it does take a bit of time to get things moving. Point the application at the directory where your source images reside and it'll scan through and create a base set of "tiles" for your mosaic. With 1,800 images it took about 15 minutes. If you're scanning 10,000 family pictures to create a mega mosaic, plan on leaving your system for a while.
Once you have your pool of pictures scanned, creating a large mosaic is straight forward. Pick your primary image, give it a name, and then select the number of images and resolution of the final image. The size and resolution of the final image is largely limited by how much time you want to devote to letting your computer chug away at cranking out an image. For our sample image we created a 2600x3300px image using 8000 images, which took around 15 minutes to generate. If you have your own tricks and tools for creating interesting mosaic images, share the wealth in the comments below. Foto-Mosaik-Edda is freeware, Windows only.