Available resources for the Swift Student Challenge

use official Apple iOS app icon

Which icon do you mean ? For which purpose ?

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can I use the screenshot images of the iMessage?

Use for what ?

Take care, there are restrictions in this domain. See guidelines:

5.2.4 Apple Endorsements: Don’t suggest or imply that Apple is a source or supplier of the App, or that Apple endorses any particular representation regarding quality or functionality.

5.2.5 Apple Products: Don’t create an app that appears confusingly similar to an existing Apple product, interface (e.g. Finder), app (such as the App Store, iTunes Store, or Messages)

So, gnerally speaking, I would avoid reusing Apple material to "promote" the app. That may fall under those guidelines.