Just use a "normal size" photo for main theme.
General rule for main theme image size:
Try not to use main images that are less then 5 times bigger in pixels than size of mosaic in tiles.
Let's suppose that you want to make mosaic with 30 columns and 40 rows:
30 x 5 = 150
40 x 5 = 200
Probably you have a much bigger photo. It's OK! Use 5 - 10 - 20 Mega Pixel shots as main photo. No
problem! You don't need to downsampling nor upsampling your Main image. Just open it with Mazaika
and mosaic it.