you need to select a simple, easy to recognize image.
to avoid a complex pictures with many figures.
you will render mosaic all tiny details will gone, only main subject
outlines will be visible. You should to select an image that don't
depend of small details.
don't use over and under exposed photos and photos with essential
details in shadow or light areas.
you had to make mosaic with two faces (a wedding photo for example) you
need to select a photo where both faces have a similar brightness.
Otherwise - probably you will end up with mosaic where one face is
"sharp and clear" while other face is not visible at all.