Often I find apps where the developers have cut the exact images they need for buttons and other graphics, sometimes with the text on the buttons as well.
OpenCV is a massive computer vision library, just have a look at its index. It might be a good idea to bookmark that link too as I am sure you will want refer to it later on in your OpenCV projects. Getting started with such a big library can seem daunting to the uninitiated so
I have recently had a project where I had to do some image processing on an image based on where a user touches an image (in a UIImageView). Getting the touch coordinates was easy enough, but the challenge was turning those touch coordinates into pixel coordinates. Depending on the way the UIImageView is set to