I think the way it came out definitely sounded a little off-putting, if not rude. People ask me to take photos all the time and I love doing it. I am not the best photographer either, but it only takes a moment and I usually show them the photo after to ask if it's ok, if I'm not in a rush. They probably asked you since you had been sitting there and even though you were reading, you didn't look rushed or "busy." It is totally fine if you really think you're a bad photog and don't want to ruin their picture, but like others said, you should just simply say, "I'm sorry, but I am really bad at taking pictures" or something simple. Saying "umm..no, thank you," sounds a little snarky to me. Next time just say "I'm sorry I didn't hear you at first. I really shouldn't because I am not good at taking pictures." I really don't feel like they were out of line to ask you. They were out of line to how they reacted afterwards, but in all honesty, the whole thing sounds weird to me. Good luck next time this may happen! Don't stress about it too much since you'll never see them again.