Usually you need to select a simple, easy to recognize image. Try to avoid a complex pictures with many figures.

When 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.

Try don't use over and under exposed photos and photos with essential details in shadow or light areas.

If 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.