The most common question I hear from our new user is the above title.
To improve your mosaic quality, We have to understand some basic stuffs.
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:
See? it looks much better now.