iphone 各个设备的分辨率和对应的像素尺寸 以及所用到的 在 Images.xcassets 中的图片规格

http://mobileoop.com/how-to-use-auto-layout-in-xcode-6-for-ios-7-and-8-development

• 1x images are for the original iPhone through the 3GS - 'standard' resolution devices (3.5" screens)

• 2x images are for the iPhone 4 and 4S (3.5" Retina screens)

• Retina 4 2x are for the iPhone 5 and 5s (4" Retina screens)

• 3x images are for the new iPhone 6+ (5.5" super-Retina [3x] screen)

http://www.paintcodeapp.com/news/ultimate-guide-to-iphone-resolutions

https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/IconMatrix.html

In 1x place image with resolution 320 x 480. 
In 2x place image with resolution 640 x 960. 
In Retina 4 2x place image with resolution 640 x 1136. 
in 3x place image with resolution 1242 x 2207. 

Images of Retina 4 2x will upscale to resolution 750 x 1334. （750 x 1334是iphone 6用）
Images 3x will downscale to resolution 1080 x 1920. （1080 x 1920是iphone6+ 用）

注意：iphone 6 使用的是 @2x的图片

解决办法 

http://stackoverflow.com/questions/25892207/how-to-specify-size-for-iphone-6-customised-edge-to-edge-image?lq=1

怎么使用 Images.xcassets

How to use Image.xcassets: 
https://www.youtube.com/watch?v=_36Y6rDcKP0&list=PLXCowKcXAVgrCe2Lezv0acRf4adQLshv2

iPhone 4S
Screen Size: 3.5 Inches
Resolution: 640 x 960 (Half: 320 x 480)

iPhone 5
Screen Size: 4.0 Inches
Resolution: 640 × 1136 (Half: 320 x 568)

iPhone 5S/5C
Screen Size: 4.0 Inches
Resolution: 640 x 1136 (Half: 320 x 568)

iPhone 6
Screen Size: 4.7 Inches
Resolution: 828 x 1472 (Half: 414 x 736)
Resolution: 750 x 1334 (Half: 375 x 667)

iPhone 6 Plus
Screen Size: 5.5 Inches
Resolution: 906 x 1704 (Half: 453 x 852)
Resolution: 1242 x 2208 (1/3: 414 x 736)
The size of iPhone 6 Plus is @3x scaling. So, it is divided by 3.