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.