I am more or less self-taught and specialize in bringing to life photographs of both people and pets in either black and white using graphite pencils, or colour using pastels.  I feel that a far more interesting portrait can be painted with the use of photos as these can often capture a typical but fleeting expression of the subject and so give a better insight into the character and personality of the person. When someone has to sit still for a period of time for a live sitting it is easy for them to become stilted and not project a true image of themselves. In the case of children and animals it is just about impossible to get them to stay still for any length of time anyway.  Also using a photo is the ideal, and only, way for people to be able to remember loved ones.