The simpler media website CMS
I'm not sure whether this is a Zenphoto issue, or one with the zpbootstrap theme I am using. The problem is that I can't find a way to zoom in on the full-size image on devices that don't have a touch screen (where I can pinch to zoom). Is there such a facility, please?
This is neither Zenphoto nor a zpBootstrap related. It is actually an option of the fancybox script zpBootstrap uses for the full image modal. That generally does work on desktop browsers as well if the image is larger than displayed in the modal.
Not being familar with the theme I don't know if there is a theme option. If there isn't you need to modiify the theme to enable that yourself: https://fancyapps.com/docs/ui/fancybox