Photo Size and Resolution for iOS app

Question

I am working on an iOS application that will show photos either in full screen or in frames in various sizes - like product catalog.

I know I am supposed to use images.xcassets to make things simple and I know I am supposed to create more than one version of the same photo in different sizes.

All I can find is AppIcon and LaunchImage guidelines. What would be the rules I should follow preparing photos to show in different devices? Sizes, resolutions?
Sample: I have 2048 x 1536 raw sized photo (even bigger ones). When it comes to prepare it for iOS app (will work both in iPad and iPhone), what are the dimensions to export my photos from PhotoShop? What would be the biggest sized version?

Could someone please show me a starting point to understand this?

Thanks in advance.

Answer 1

This design guidelines guide is very good. You will find everything you have to know about sizes.

But the simplest way is to use a tool like Asset Catalog Creator to generate your assets. Just drop your original image in it, select your @1x size, and it will generate the catalog for you, for all sizes.

Answer 2

Here is a screen size reference for iPhone devices you may find helpful. Here's one for iPads .

Additionally, if you've got a lot of files you need to resize that you've already exported, you might consider using ImageMagick for batch resizing.