How to crop one photo into another?

So I've seen people crop a picture of someone onto another picture… So what's a free app for iPhone that I can use to take 2 of my photos and crop one into the other (not blending but cropping)

You need to use a professional type tool on a computer, you are going to be doing an app for that unless you want it to look cheesy

Crop cut out is usually selection with lasso tool or scissors, and copy /paste over second image,
To save requires transparency channel as png or tif, if not, you use save as to clipboard,
Check apps for Apple like
http://www.apple.com/...-apps/ios/

Procreate app
https://itunes.apple.com/...73498?mt=8
Superimpose
https://itunes.apple.com/...13585?mt=8
ISwap
https://itunes.apple.com/...54030?mt=8
.