When an image is opened in the image viewer, it would be great to be able to double tap and pull down / up to zoom in / out as is the case in many other apps, at least on Android.

This would make using the app with one hand much handier.

Edit: to clarify, I don't mean the existing gesture which zooms by a fixed amount after double tapping. I would like to be able to hold the second tap and then pull up or down to manually zoom out or in.