The following resolutions are acceptable to iTunes connect:
- iPhone 3+4 (3.5 Inch)
- 640 x 960
- iPhone 5, iPhone 5S, iPhone 5C (4 Inch)
- 640 x 1136
- iPhone 6, iPhone 6S, iPhone 7, iPhone 8 (4.7 Inch)
- 750 x 1334
- iPhone 6 Plus, iPhone 6S Plus, iPhone 7 Plus, iPhone 8 Plus (5.5 Inch)
- 1242 x 2208
- You need the screenshot in this resolution, the phone scales them down to 1080 x 1920
- iPhone X (5.8 Inch)
- 1125 x 2436
- iPhone XR (6.1 Inch)
- 828 x 1792
- iPhone XS (5.8 Inch)
- 1125 x 2436
- iPhone XS Max (6.5 Inch)
- 1242 x 2688
- iPad Mini 2, iPad Mini 3, iPad Mini 4 (7.9 Inch)
- 1536 x 2048
- iPad 3, iPad 4, iPad Pro, iPad Air, iPad Air 2 (9.7 Inch)
- 1536 x 2048
- iPad Pro (10.5 Inch)
- 1668 x 2224
- iPad Pro (12.9 Inch)
- 2048 x 2732
- Apple Watch Series 1, Apple Watch Series 2, Apple Watch Series 3 – 38mm (1.5 Inch)
- 272 x 340
- Apple Watch Series 4 – 40mm (1.57 Inch)
- 394 x 324
- Apple Watch Series 1, Apple Watch Series 2, Apple Watch Series 3 – 42mm (1.65 Inch)
- 312 x 390
- Apple Watch Series 4 – 44mm (1.78 Inch)
- 448 x 368
Hope this helps. Even Apple’s Documentation on the matter is incomplete.
Update: Apple has introduced a new Media Manager which requires only that you create artwork for the largest iPhone and/or iPad devices. The smaller images will be created for you. Note that if you hadn’t been creating iPad Pro images before, you have to now (if you support iPad).
Update: Screenshots and app previews for new devices now supported.