When a portrait is commissioned there is an assumption made the image will look like the subject. Here is where the discrepancy between 'art' and 'rendering' come into play.
If it is a photograph you want then go to your local camera shop where there is a service that will transfer you favorite photo onto a stretch canvas. There are photoshop filters that can be applied to make it look like it was painted onto the canvas. This is a form of art I like to subcategorize as a rendering. The subject is rendered exactly as they look.
Depending on style, medium, and skill; art should have some emotional impact on the viewer no matter how close the resemblance a painting has to its subject.
For the artist:
If it is a rendering the patron wants, then get out the tracing paper and projector. If it art they want, then be consistent with your style and paint with passion.