Have a look at the Now Playing tab > Large Album theater mode (or any of the other default two).I tried it, this option is equivalent to "search internet for picture" and downloads small size covers instead of full size covers as displayed in theater mode. This is a very rare album, and I can't find a full-size on internet either and surprisingly theater mode is only place I can see it in full size. So how does it upscales it?
Right-clicking on the artwork there will allow you to download it.
This method downloads the image you are viewing, not a small size version (and is most likely one of the images linked to by sveakel in the previous post).
The Theater Mode view shown in your original post (Artist Pictures) uses the 'zoomKeep'
property for the images displayed which means that it will zoom in on any image to fill the height of the screen whilst maintaining the aspect ratio of the image.