This article explains how to get full-screen photos for incoming iPhone calls. It covers setting full-screen contact photos with both new photos and existing contact photos. The instructions in this article apply to iPhones running iOS 8 through iOS 16.
How to Enable Full Screen Photo for Incoming Calls for Contacts with Caller ID on iPhone and iOS
If you add a new photo for a contact on your iPhone, it's easy to make it full screen for incoming calls. Simply add the photo to the contact by following these steps:
Open the Contacts app and tap the contact's name.
Tap Edit on the contact details screen.