Already thought of using a mirror?
No really. You could make your painter sim 'remembering a scene' by using his or her mobile phone. So place all your sims together in a nice pose, with free will off and hideHeadlineEffects on (thank you Bubbo) and than let that sim take a picture of the mirror that is in front of the family.
Now he can make a portrait of everyone including himself.
Let's say, in theory this must work. I've never used it in 'real sim life' actually. Hope you can make this all true.