I occasionally get such requests, so it's probably legitimate, but you should charge for this kind of usage. Consider that they could just go to a stock photo site and license an image like this fairly inexpensively, so they're being really cheap by trying to get it for free. You might at least see what the stock agencies are charging for such usage and offer it to them for that much, and since it would be a direct sale you don't have to split it with an agency.

Alamy has a pretty good calculator for determining the price of an image for various kinds of usage, so you might go to alamy.com, search for a similar image (maybe find one image that's Royalty Free and one that's Rights Managed for comparison), then plug in all the parameters and see how much it would cost you to license it for the same purpose, and charge that much.