Anna from Serendipity gave me the answer to yesterday's question. The pictures are called photograph mosaics or photomosaics.
I'm sending Anna a fabric postcard to say thanks!!!
This is what I was talking about...
(you can download these as wallpaper for your desktop at: http://andreamosaic.org/wallpaper/)
Now that I know what they're called I've found heaps of links.
This one is particularly impressive - Infinitely zoomable photographic mosaic - its a photomasaic that just keeps going on and on and on. When I saw the first picture I couldn't imagine that it was a mosiac, but I was wrong!! note: it needs a flash player to view.
There are also heaps of programs that let you create photographic mosaics using your own photographs. My head is reeling with the possibilities!!