We visited YNP & GTNP in June. I took HUNDREDS of photos and one of the tips I would share is to take photos of signs in Yellowstone. i.e. there are lots of labeled geyers... take photos of the signs seperate from your photos so when you create a digital album you can put captions on your photos. Also take photos of the map signs. For example in the Mud Volcano area I took a photo of the wooden sign that is at the beginning of the boardwalk that shows all the names of the various mud volcanos. I wish I had taken more photos of signs because I only thought of this mid vacation and only have a few.
If you are the creative type some of my favorite pics are the zoom shots of the cracks that create mosaic designs at the mud volcano area. These are going to look great as framed black and white shots.
One final tip...please invest in a camera with a large zoom!!! I can't tell you how many people I saw in the park with dinky nonzoom cameras or using their cell phone cameras. Most people visit nat'l parks once or twice in their lifetime...capture it with a GREAT camera and something more than a 3x zoom!
And of course take more photos than you'll ever need and numerous shots of the same thing, different angle. You'll be glad you did.