use a "normal size" photo for main image.
rule for main theme image size:
not to use main images that are less then 5
times bigger in pixels than size of mosaic in tiles.
suppose that you want to make mosaic with 30 columns and 40 rows:
5 = 150
5 = 200
you have a much bigger photo. It's OK! Use 4 - 6 - 10 Mega Pixel shots
as main photo. No problem! You don't need no downsampling nor
upsampling your Main image. Just open
and mosaic it.