On May 17 I had to go to the GR inside the park, too. I tried at the one outside the park in case that was is (it wasn't morning, but I didn't want to miss it and then have to go out and back in), but they told me to go to the one inside, which is right next door to them. Outside it's a window, but inside it's in a building, at a counter.
The camera is set up as you're coming out of the mountain one time, as you're about to go down a hill. It may be right after you see the Yeti, but I'm not positive.
You get the coupon by playing a game that you download from Disney called Yeti Vision. You win virutal prizes for VMK and then after level 3 you get a pdf file that you can print out to get a coupon for the photo. The offer is only good through September 30, 2006.