Yeah but pdf are Portable Document Files...not graphic files. Tell me, are you using the free pdf reader that comes with 8.1 or are you using Adobe?
I ask because I always use Adobe for pdf files and Calibre for most other formats (though it works with pdf files as well. With the free Calibre reader one can download and install cover art for the books one has converted for the Calibre Reader (those
that didn't include the cover already) and THESE will show up in the preview pane.
The free Adobe Reader (I suggest getting the 10,1,13 version as the Adobe XI has a lot of problems)
you can get here: http://get.adobe.com/reader/otherversions/
And the free Calibre Reader can be found here: http://calibre-ebook.com/download_windows
And, of course, the free Kindle for PC from Amazon kicks...uh, I mean, it is very good.
GET
FREE KINDLE FOR PC
(If I remember right there's also a Kindle app in the Apps that come with 8.1 but one cannot adjust the size of the text for aging eyes which is why I got mine direct from the Amazon store and uninstalled this one).