I'm not aware of an extension that allows to do that.

Clicking an image normally would make Firefox perform the action that is attached to the image if there is such an action, either because the image is a link or there is an onclick event attached. An extension would only be able to do what you want if there is no such action attached to the image.

Note that you can middle-click context menu items like "View Image" to view the image in a new tab instead of in the current tab.

So there's no way to shortcut right click -> view image? Meh