Who the heck caares about the credit? What good will that do you? It seems amateurish to need to see one's name on things.
The prices being reccomended here are largely outrageous. The ad person knows they can just go to iStock and get the rights to use an image for a couple dollars. Why the heck would they ever pay $300, $500, $2000. Yes, these are prices that people on here actually suggested charging.
The way to get the most $ may be to offer a package deal for a number of images you took at that zoo. Give them unlimited rights to use the images for one year and charge a few hundred bucks. They'll probably just ignore you, but at least if they do take you up on it, you'll get a few hundred bucks out of the deal. I doubt they'd pay any more than $50 for the one image you mentioned. Actually, they'd probably only pay about $25 for it.
Another thing - by selling them the rights to use the image, you are in no way giving anything up. Sell it as unexclusive rights, meaning that you still may use & sell the image in any way you care to.
Tom -- the only part I will disagree with is the pricing. If the person contacting the OP would have wanted a stock image they would have gone that route. They contacted the photographer for a specific image that is not a stock image.
I have had people contact me for images for commercial use-- I typically price them at $500 for organizations like this-- the majority of the time the $500 is a fair price to them.


