Right now, I use coupons solely as a discount rather than as a way to entice people to buy my images (though, admittedly, the difference is subtle). For instance, I have a group of friends whom I'm offering a discount. I basically sent them an email saying "hey, you can download photos here, and they're cheaper if you use this coupon code." I'll probably do the same for family members.
In my experience both as a shopper and watching other folks shop, limited-time offers can be a great motivator to get people to make the "buy" decision for an item that they're on the fence about. Especially so if there's a minimum price to activate the discount (which Zenfolio doesn't currently support).
Anyway, hope that gives you some ideas. Cheers,
--xsdg