Tolerance for zoom-button
The zoom-button shows as soon, as a zoom is possible. It still doesn't make sense to zoom in any case, though. For example, if only a few pixels can be zoomed in, it is no advantage.
I suggest to add an option "zoom tolerance" in pixels or percent, which allows for to adjust, when the zoom-button should show (for example only if the image can be zoomind in by more than 20 %).
Note, some people might want to zoom the image in larger, even if no higher resolution is available.
If you are on a very high resolution screen, sometimes you just want to see things bigger.
I would prefer the option to zoom the image to say 2000px wide even if the image does not actually have that many pixels.