Using Guilt Instead of Cryptography

Yeah, let's implement this, and now everyone has to think twice before buying an app because they see a dollar bill leaving their hand rather than some abstract number.

It'll be great for sales!

I think it's a great idea, but doesn't an abstract representation of cash also benefit the merchant, because it makes people more likely to make spur-of-the-moment purchases? Making this process much more concrete and emphasising the "taking your money away from you" aspect might make everybody less inclined to pay for things and thus might make a payment system based on this idea unpopular.

I like the camera idea. It would probably work just as well if the unauthorized user merely believed a picture was being taken; just have a pop-up message saying so before the transaction is finalized and skip the picture altogether.