Superimpose (US$0.99) does a first class job of taking part of one photo and adding it to another. It's great for removing your best friend from a picture taken in your living room and putting them at Niagara Falls, or taking the head of one person (or your dog) and placing it on another body.
There have been other apps that do this, and of course on your Mac there are Photoshop or other image editing applications, but Superimpose makes it easy and has the added advantage of giving you tools to match the exposure and color of the two layers before you blend them.
To use the app, click on the Library button and you are asked to load the background, followed by the foreground. There are a variety of masking tools that let you knock out the foreground areas that should be eliminated. One handy tool is a Magic Wand that tries to do this automatically. That's only likely to succeed if your subject is standing against a solid color or the sky. For further masking tasks, there are brushes that can be resized to get as little or as much as you want. One thing the app does very nicely is that it offsets the view of what you are masking, so you can see the image without your finger being in the way.
If you miss on your masking, there is an eraser tool that lets you un-mask any parts that are sloppy. With a little practice, you'll have the best methods down. Superimpose allows you to rescale, rotate or flip the foreground to work best with the background.
To make things even easier, you can match the color cast of the two layers. Nothing screams composite louder than two pictures with different color appearances. There is even a tool that lets you put a shadow of the foreground object on the added background. Superimpose also supports blending of textures and there are a variety of filters for special effects. Your saved images will be at full resolution, giving you the best possible quality.
I had no trouble at all using the app, but it does take a bit of practice to learn the art of masking. The tools provided make it as easy as it can be, and you are likely to get great results.
Superimpose is not an app you will use every day, but when you want to try the effect it's one of the best apps I've seen.
The app contains links to video tutorials, which I would recommend you view before you jump in. Superimpose is a clever app at a reasonable price. It requires iOS 7 or later, and it's universal.