I've seen other photomosaics posted here and frankly, I could never get them to work right :-( So I thought I'd share a simple idea I had for my wife's 40th birthday DVD intro.
I used a free photomosaic software found at:
http://www.andreaplanet.com/andreamosaic/ and then I put it all together in Sony Vegas with other photos moving (and dropping into) the larger photomosaic.
This is my first, shorter, rough draft, (the final was longer, with a lot more photos and different music) but you'll get the basic idea I was going for. You can watch it and download the .veg file here:
http://www.arrfdesigns.com/photomosaic_intro.html