Steemit picks the first image to be your cover/thumbnail, but it will be blank if the URL of the image has parenthesis ( ) in it. It's a known bug and may be fixed eventually, but for now, don't choose an image that has parenthesis in its URL.
Great intro and follow up article on $10 steem! Followed. Best wishes!
Kenny thank you for helping me figure out how to avoid the bug for the thumbnail not showing!