PS：很基礎的東西，記錄一下，用起來方便些。

按iOS版本分：
iOS5，6：320 x 480(1x) 、640 x 960(2x)、 640 × 1136(Retina 4)
iOS8，9：1242 × 2208(Retina HD 5.5″)、 750 × 1334(Retina HD 4.7″)
iOS7-9 ：640 × 960(2x) 、640 × 1136(Retina 4)
iOS11+：1125?×?2436（3x）。蘋果X的。

所以，4/4s需要2張。5/5s需要2張。6/7/8需要1張。對應的plus需要1張。iPhoneX需要1張。（3GS可以忽略了）共計7張圖片。
對應Assets.xcassets文件夾中的Contents.json文件。其中filename需要和圖片名一致。

{
  "images" : [
    {
      "extent" : "full-screen",
      "idiom" : "iphone",
      "subtype" : "2436h",
      "filename" : "startpage_iPhoneX_1125x2436pt@3x.png",
      "minimum-system-version" : "11.0",
      "orientation" : "portrait",
      "scale" : "3x"
    },
    {
      "extent" : "full-screen",
      "idiom" : "iphone",
      "subtype" : "736h",
      "filename" : "startpage_plus_iOS89_414x736pt@3x.png",
      "minimum-system-version" : "8.0",
      "orientation" : "portrait",
      "scale" : "3x"
    },
    {
      "extent" : "full-screen",
      "idiom" : "iphone",
      "subtype" : "667h",
      "filename" : "startpage_iPhone678_iOS89_375x667pt@2x.png",
      "minimum-system-version" : "8.0",
      "orientation" : "portrait",
      "scale" : "2x"
    },
    {
      "extent" : "full-screen",
      "idiom" : "iphone",
      "subtype" : "retina4",
      "filename" : "startpage_iPhone5_iOS789_320x568pt@2x.png",
      "minimum-system-version" : "7.0",
      "orientation" : "portrait",
      "scale" : "2x"
    },
    {
      "orientation" : "portrait",
      "idiom" : "iphone",
      "filename" : "startpage_iPhone5_iOS56_320x568pt@2x.png",
      "extent" : "full-screen",
      "subtype" : "retina4",
      "scale" : "2x"
    },
    {
      "orientation" : "portrait",
      "idiom" : "iphone",
      "filename" : "startpage_iPhone4_iOS789_320x480pt@2x.png",
      "extent" : "full-screen",
      "minimum-system-version" : "7.0",
      "scale" : "2x"
    },
    {
      "orientation" : "portrait",
      "idiom" : "iphone",
      "filename" : "startpage_iPhone4_iOS56_320x480pt@2x.png",
      "extent" : "full-screen",
      "scale" : "2x"
    },
    {
      "orientation" : "portrait",
      "idiom" : "iphone",
      "filename" : "startpage_iPhone3GS_iOS56_320x480pt.png",
      "extent" : "full-screen",
      "scale" : "1x"
    }
  ],
  "info" : {
    "version" : 1,
    "author" : "xcode"
  }
}