What is the ideal size for Main image?
Please
use a "normal size" photo for main image.
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
I.e.
any picture starting with as small as 150 x 200 pixels and bigger is
suitable for the Main image. Simply select
an image where all essential details are clear visible.
There are no special restrictions on picture size.
Probably
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.