I found somethings about scaling on iPhone 6 plus : "For the iPhone 6 Plus, Apple has introduced even larger “3x” assets that are three times the size of things on non-Retina devices. This helps keep things visible on the larger, denser screen without making buttons and other tap targets too small to work with, and without stretching 2x assets out and making them blurry (though in these early days, most apps will be a little blurry anyway). To balance size with sharpness, Apple is doing something a little weird here. Rather than displaying things at the phone's native 1080p resolution, the iPhone 6 Plus is actually rendering a 2208×1242 image and downscaling it to 1080p."
Should I need to make my images 3x or 4x bigger, I'm using 2x images ?