Apple's native Photos app limits the ability to zoom on photos beyond a certain threshold, using its built-in pinch to zoom feature. However, the folks at iOS Jailbreak Pro have identified a bug in iOS which enables users to override the limit of zooming in on photos.
Here is how to exploit the iOS bug for zooming unlimitedly on a photo:
1) Open the Photos app on your iOS device and tap on any photo.
2) Choose Edit and then tap the Crop tool. Now rotate the photo by 90 degrees by hitting the Rotate button.
3) Finally, hit Done to save the changes.
That's it. You should now be able to zoom unlimitedly on any one photo at a time.
Note: Quitting the Photos app or switching to another photo will stop the unlimited-zoom bug from working. You will need to rotate every photo that you want to zoom unlimitedly.
It is confirmed that the unlimited zoom works flawlessly on any iPhone 6s running iOS 9.3.2.
If you are still sceptical, check out the demo video (below) depicting the bug in action (courtesy iOSJailbreakPro - in Spanish):