In this tutorial, we explained what is main image and cell image, and all ops of the tool panel.

Photo Mosaic is a MOSAIC. it is a kind of low resolution graphic. This means, in most case, mosaic is not designed for carrying details.
For a common poster size project, 20×24 inch, the maximum cell count could reach 80×96 cells. Which means your main image will be represented by 80×96 pixels.
Lets take a look at the following main image.
This main image contains many details, lets see how it work on the final mosaic:

Fuzzy? yeah, we can see there are only few cells on the face ares, so it is hard to recognize these people.

The solution is crop the main image, for example, we can use the top side only.

and now check the output:

See? it looks much better now.

So,  keep in mind, for most poster size mosaic, keep your main image no more than 3 faces.
for postcard size mosaic, you’d better use a single face photo, no more than two.