OP, I think you were fine and the lady & teenagers were quite rude. I agree that your response may have been confusing, but you were coming from book-brain and were confused yourself. I don't blame you for not wanting to step away from your books or your luggage, and I especially don't blame you for not wanting to handle someone else's camera.
Personally, if I want pictures of myself & whoever I'm with, I either try to ask an employee of where I'm at (if possible), or I make do with friends & I passing the camera around and taking pictures of one another. I don't think it's rude to ask a stranger once, politely, to take a picture (provided said stranger isn't obviously occupied with something, and I do thing reading counts as occupied with something), as long as an answer of "No." is taken gracefully.
I don't mind doing favors for people, but it's going to be at *my* convenience and at *my* comfort level, not theirs. For strangers, anyway. I'm more willing to be put out when it's family/close friends.