In order to close an image field zoom overlay, one has to click the image itself, and clicking the dark overlay background has no effect. However, it's generally common in such viewers that clicking the background closes it, and clicking the image itself doesn't.

It's especially problematic because right-clicks close the overlay too instead of opening a context menu (e.g. for downloading the image).

In addition, a close button ("x") might help guiding inexperienced users.