When you use Share Screen in Zoom and you select a single app like Photo Mechanic, Zoom will not share any new windows that the app creates. So when you open a Preview window, Zoom viewers will not see it. You can get around this by sharing your entire desktop, or you can hold down SHIFT and share multiple windows.
The trick is that you can multi-select Photo Mechanic and a Preview window, then completely close the Preview window. When you open a new Preview window, Zoom will remember and then display the new Preview window as expected.