iOS
6.5.5-Inch Retina Display
1242 x 2208 pixels for portrait
4.7-Inch Retina Display
750 x 1334 pixels for portrait
3.5-Inch Retina Display
640 x 960 pixels for portrait (full screen)
12.9-Inch Retina
2048 x 2732 pixels for portrait
9.7-Inch Retina Display
1536 x 2048 pixels for portrait (full screen)
android
JPEG or 24-bit PNG (no alpha). Min length for any side: 320px. Max length for any side: 3840px.