Try the tips in this article. It was recently updated so should work but I have not tried them. Quite simple though. http://bit.ly/2FMveoo
[doublepost=1516205907][/doublepost]Actually I just tried it. Very simple. Go to the contact card, tap edit, then tap edit under the photo. You’ll get options, just choose edit. Just move the picture in the circle a tiny bit and leave it. Tap done and done and that’s it. Had my wife call me and it was full screen.
It should automatically be full screen if the resolution is big enough. The problem is that contacts can bug out the image (I assume it happens due to iCloud sync) and lose the circle position + the full screen setting.
I go through my contacts once every while to check whether they are still fullscreen (can easily be spotted via the circle image, if it‘s no longer focused on the part you set it to then it‘s reset).
I actually hadn’t seen a full screen image for a call in years. It wasn’t until I read this post that I remembered I used to, way back when. That led me to searching out that little trick which does work. It’s not a biggie but can be useful and there should be a setting for it since it still works.