is a photo enough to effectively target a specific person with a spell or sigil? If So how do I direct the magick?

It depends on the spell/curse; there are those that also require the target's full name, dob, parents' names, personal belonging, body parts (nail clipping or lock of hair etc), or others.

People often use photographs of the intended target for curses, usually writing the name of the individual on the back & either burning the photo during the ritual or placing it in a hex jar. If you are the kind of person who can focus on a person's energy using a picture, then a photograph will most likely be sufficient, if not, you might want to obtain something belonging to the person like a lock of hair, article of clothing, etc.