How appropriate is it as a single guy to put his arm on a married woman's waist during a photo? Does this count as a friendly gesture or inappropriate behavior?
If she's not given you permission to touch her, you likely shouldn't. This is a pretty intimate touch - why do you feel it's needed? What do you gain from it, how does it make the photo better?
Your best bet, if you really feel you have to do this, is to ask her what she's comfortable with - as this is going to vary from person to person. Whatever she says, respect it.