Zoom is a transient action, if you want to look closer into some detail of an image. Zoom does not impact the overall size of the image relative so as you zoom in and out, the annotations you have or add will also be zoomed in or out. Zoom is controlled by the magnifying glass in the image active window.
Scale is a persisted action. With Scale you are changing the overall dimensions of the image so you are changing the relative size to the annotations. The Scale is accessed via the Edit icon where you also can crop the image. The Scale can be adjusted by dragging the corner of the image in or out, or via a text input.
When a user adds an image we potentially both Scale and Zoom the image to give them the optimal user experience. We scale the image so that if it is large we would display it at an appropriate size on a typical monitor. We then Zoom the image to make it displayed well on that particular users monitor and window size. If they change the window size, we continue to zoom the image. If the user subsequently changes the scale then that change is kept as the new scale.