Make one large image by tiling many smaller images in specified order?

[jupiter]

How do you get GIMP to make one large image out of about 6,800 small (256x256 pixel) images? It's like you had one large picture and you cut it up into 6,800 little squares. I only have the little squares, not the original, and I want to put them together without any seams showing to make the large original image. They're numbered in sequence, they must stay in the exact sequence, and I already know exactly how many rows and columns the composite needs to have. If it's easier in some other program than GIMP that's fine too.