Personally, I think you have to adopt a simple stance.
That is always say yes, first, but request purpose and usage, exactly as would Getty or Corbis, thus indicating that a charge is naturally a matter of course.
You might expect your favourite bar to give you the odd - very odd! - freebie, but you wouldn´t expect that from a bar to which you´d never been before, would you?
In both cases the product is product is business.
How complex, simple or different a painting and photograph are has nothing to do with it; the barter is done via money or some other valuable consideration which can be related to a monetary/measurable value that suits BOTH parties or there is no deal.
I suspect that most professional photographers or artists would find no problem with the concept of valuable exchange - that´s what all selling´s about. The worst attitude is where the person being asked feels flattered and becomes stupidly generous for that reason. It kills the profession. If that doesn´t matter to you, then give your work away - it´s only you being creamed, despite the collateral damage you do the rest of the photographic world which, no doubt, couldn´t bother you less. (This is NOT directed to the OP; it´s a general statement.)
Rob C