Whatever the joke is has to be funny, and not coming from a mean-spirited place. I think some things are totally off limits. If someone's spouse died, or one of their children, I would never joke about that. I don't have any aspirations towards writing any cancer jokes, and there's some stuff that I think is definitely taboo.
Every image is in some way a “portrait,” not in the way that it would reproduce the traits of a person, but in that it pulls and draws (this is the semantic and etymological sense of the word), in that it extracts something, an intimacy, a force.