When adding graphics to your app, you want to make sure they'll be the right size. Our recommended dimensions for image banners, backgrounds to your home screen and list banners in your media sources are:
Image Banners (home screen):
Rectangle - 2048x700
Backgrounds (home screen):
1536x2048
Otherwise, keep it under 1MB
List Banners (in a media sources):
Rectangle - 2048x600
Square - 1080x800
We recommend compressing the images so it loads faster on devices with a slower connection. You can either use software like photoshop or compress them online using Tinypng.com