I've been approached by a designer wanting to use one of my images for promoting an exhibition, and she has asked me under what terms they can use the image (or not at all, of course). I have no idea what would be reasonable.
They want to make promotional leaflets/handouts as well as banners like the ones in the image below (not my image), and want them up for a period of 4 months. The exhibition will be set up by a public/government run museum and will be "educational" if that makes any difference. I'm not dealing directly with them, though.
My main concern is to not ruin the market for the professionals out there - for me it's purely a hobby.