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 wouldnīt expect that from a bar to which youīd 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 - thatīs what all sellingīs 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 doesnīt matter to you, then give your work away - itīs only you being creamed, despite the collateral damage you do the rest of the photographic world which, no doubt, couldnīt bother you less. (This is NOT directed to the OP; itīs a general statement.)