A good quality photograph (clear and sharp) that captures the character of the subject is essential for a perfect result.
Photographs taken from mobile phones are generally not good enough unless a minimum of mega-pixel camera is used. The higher the better of course and
most modern digital cameras provide an even better definition which is ideal.
If you are not sure which one to choose I would be happy to advise you (sometimes a not so obvious lighting of the subject or a contre-jour can provide a great opportunity to enhance the quality of a picture at its best).
Avoid using flash photography
Casuals, off guard photos often make more interesting portraits than posed shots.
The chosen subject can also be selected from a bigger scene (group etc) as long as when isolated it keeps its high quality definition.
Backgrounds are of no importance.