I have to say that I hate that whole "why make the world colder" line of argumentation, Flora Louise. It comes across as a huge guilt trip. It's like people who say to do something for a demanding relative because "you're lucky to have relatives". All it does is line the road to doormattery in the name of niceness.
I hope you're kidding. We are talking about taking a photo, aren't we?
Actually, I'm not kidding. I find those kinds of arguments to be specious as a whole.
If all we're talking about is taking a photo, then it shouldn't be a big deal that someone declines to take it, right? So why is it only the asker who is taken into consideration by this guilt trip? I take horrid pictures; therefore, I always decline to take photos for others -- what kind of kindness is it to hand them over a photo of my thumb?
Exactly--they think they have a memento of the trip, and therefore not ask anyone to take another, then later discover they have a thumb picture.
Whereas if you just said no, they could ask someone else and get a picture, and then they'd have one.
(It's less of a problem with a digital camera, since they can see right away how it turned out, but in the days of film it was horribly disappointing to think you had a good shot and it didn't turn out...)