As per title, 

Is it possible to zoom out a cover photo? The height being less than 200px is quite short (maybe this is just my theme) I can adjust the css for cover photos which is okay but ideally it would allow users to resize (based on zooming in and out as well as moving around) the photo.

No, I believe the image width will automatically stretch to match up to the cover size width.  

Keep in mind that you prob want cover photos with a higher Height x Width ratio because the cover photo is "fatter" on mobile than desktop.