图片所需要的大小:
iPhone Portrait iOS 8-Retina HD 5.5 (1242×2208) @3x
iPhone Portrait iOS 8-Retina HD 4.7 (750×1334) @2x
iPhone Portrait iOS 7,8-2x (640×960) @2x
iPhone Portrait iOS 7,8-Retina 4 (640×1136) @2x
iPhone Portrait iOS 5,6-1x (320×480) @1x
iPhone Portrait iOS 5,6-2x (640×960) @2x
iPhone Portrait iOS 5,6-Retina4 (640×1136) @2x