Closing an image is not intuitive (on iOS app)
Feedback from non-technical users I've shared a Sway with: tapping on an image while in full screen mode is not intuitive to close the image and return to the main Sway. Consider adding a "Swipe" back\forward feature OR allow the use of the browser back button in addition to tapping to close.
Those are some great suggestions to the issue you brought up. Thanks for the feedback!
Or a small "X" in a corner of the magnified photo to return to the presentation. Many programs and apps do this, it is intuitive.