This script takes an iPhone 6 plus image (1242x2208) and generates all the other phone sizes, making them ready for upload in the App Store.

Your steps would look something like this:

• Run the simulator for iPhone 6s Plus and hit CMD + S to save a screenshot or create an image that is 1242x2208.
• Run appstore_resize.py with that image and it will generate all the proper image sizes for iPhone 4, 5, 6.
• Images are ready to upload to iTunes Connect.