1. He looked at himself in the mirror.

(Does 'in the mirror' modify 'himself' or 'looked'?)

2. I had my picture taken by the photographer.

3. I had the photographer take my picture.
(Are both grammatical? Does #2 can be changed into #3?)

1. Does "in the mirror" describe him? Or does it tell where he looked?

Yes, 2 and 3 mean the same thing.