How Do You Get A Thumbnail Image To Display?

I'm trying to help my wife post to Steemit.  She has an image which displays properly at the start of her story, but is not displayed as a thumbnail when browsing articles.

What am I missing here?  Is the thumbnail created automatically from the first image or is there some code we should include in the post to generate a thumbnail?

I have searched and this question seems to come up a lot, but none of the posts I've found have a clear answer to the question.

Thanks for any feedback on this.