ScaleGestureDetector
When learning a new concept I don't like using libraries or code dumps. I found a good description here and in the documentation of how to resize an image by pinching. This answer is a slightly modified summary. You will probably want to add more functionality later, but it will help you get started.
Animated gif: Scale image example Layout
The ImageView just uses the app logo since it is already available. You can replace it with any image you like, though.
-- layout.xml