The problem might be from FB.It will grab an image from a link, and even if the image changes, it shows the old one. Try this. Click this link: https://developers.facebook.com/tools/debug/og/object/ Paste the link into the box, and then click "fetch new scrape information" Scroll down to the bottom and you'll see what FB says it will display when you enter the link the next time. HTH.
Sometimes it's not that easy. FB caches images, so the second time you post a link, it's not actually "grabbing" the new image from the source, it's re-posting the same information it collected and stored the first time. If that is the reason the correct cover isn't showing, navigating to the link I provided tells FB to clear the cache and grab new data. ( Also keep in mind, that FB doesn't "see" any images less than 200x200 pixels)