iPhone 4, 4S, 5, 5s, 6, 6s use @2x images - even though the 6 and 6s have a higher resolution screen, they have the same pixel density.
iPhone 6 Plus and 6s Plus have a higher pixel density and use @3x images - however, the screen is rendered at a higher resolution (1242 x 2208) and then downscaled to the device's actual screen resolution of 1080 x 1920.
You can use a combination of autolayout and Xcode size classes to achieve your desired layout on the various screen sizes without changing your images - for example, users on the iPhone 6(s) and 6(s) Plus generally assume that apps will show more content on their larger screen rather than show larger graphics.
iPhone 6/6 Plus screen information: http://www.paintcodeapp.com/news/iphone-6-screens-demystified
iPhone screen resolutions and pixel density: http://www.tekrevue.com/retina-display-comparison/