In what situations would it be appropriate to include one's photograph in a CV?
In the United States, you should never include a photo in an academic CV.  It comes across as somewhat inappropriate, like you are deliberately drawing attention to your appearance and hoping it will influence the decision.  (I know that's not actually the intent, but many people's gut reaction upon seeing the photo will be "Why is the applicant showing me this?  Do they think I should know what they look like before making a decision?  How is that supposed to be relevant?")  If you are from a country in which photos are often included, then that will be understood as a reasonable explanation, but it will stand out as foreign.  If you aren't from such a country, then it will look bad.