Create and save in a folder all icons formats required by Xcode in PNG with transparency. In order formats created are: 16px, 29px, 32px, 40px, 48px, 55px, 50px, 56px, 57px, 58px, 60px, 64px, 72px, 76px, 80px, 87px, 100px, 112px, 114px, 120px, 128px, 144px, 152px, 172px, 180px, 196px, 256px, 512px, 1024.
Of course the master image must be square and have a resolution (recommended) at least 1024x1024 or higher. If an image was provided too little larger sizes generated would be poorly defined.
