I will admit I am not the biggest fan of Photoshop but I am into all of these compositing apps in the App Store and this new application, Juxtaposer, [iTunes link] is one you can really have some fun with, especially if you like taking pictures of your friends and family and making them look ridiculous.
Along with all of that fun comes ease of use as well:
First, load in any two pictures - perhaps one of your dad, and one of your dog. Next, use your finger to erase around your dog's head. Quickly zoom in and out or move around using two finger pinching gestures. To cut out your dog's head exactly, first zoom out and make a rough outline, then zoom in, and erase or un-erase with pixel-accuracy. Switch to move mode and use two fingers to move, re-size and rotate your dog's head into place over dad's. It's easy, intuitive and fun, and you'll have a great new Dog-Dad in about a minute.
The app will run you $2.99 but for that price you are getting the best compositing application in the App Store. Period.