Thanks Halcyon but I think I have confused you because that's not quite what I mean. The mosaics I'm referring to are formed from many smaller but different photo images to create a large image. So, for example, you could take a picture file from your PC and recreate that image using many much smaller photo images from a folder (or folders) in your PC.
See
here for more.
I just really want to find some good, reliable but free software to do this.