Seeking the reproduction of some details may lead to failed work. A mosaic is ... a mosaic. Neither a photo nor a painting.
The theme of the tiles
Another consideration needed in the reproduction of detail is the theme of the tiles. Stay away from the screen and observe the two mosaics from below:
In the first mosaic the "roses" theme - with basically circular shapes - makes you lose the eyelashes detail, which is not the case when the tile subject is fauna stamps (with irregular shapes). It's possible to achieve the reproduction of subtle details with regular tiles, but that requires huge mosaic prints.
Conclusion: For regular size posters, if you use tiles with regular figures, mix them with irregular tiles.
The tiles size
I previously said that it was desirable to reproduce details using tiles with irregular shapes. This is necessary but not enough: we must also have in our library images with formats other than the square ones.
You can see that in the left mosaic above the girl eyes detail is best achieved. Below, the zone's description for each mosaic:
Of course, always it's possible to increase the resolution of the mosaic always using more tiles, vertically and horizontally. Below, the same painting with different tile densities, though same 3x2 format::