For any posted link, Facebook crawls the link once and then caches the data it found, including the thumbnail image to use. So if you change the cover art of your Smule performance later, new and old links on Facebook won’t use the new image. 

But there is a solution! Go to this Facebook page and paste the song URL: https://developers.facebook.com/tools/debug/

Let the system check and then “scrape” the URL until you see the new cover art. Once the preview looks fine, you can share your performance on Facebook and it will use the new cover art from now on.