Never mind – I found it. For benefit of other newbies like me, here’s what worked:
Edit the post itself. Underneath the text area and all the SEO stuff, Post Options, etc., — down at the bottom, you’ll come to Featured Gallery. Click on the blue Add to Gallery button, then select the image you want to use from your media gallery that will be displayed. Remove the extra one that came with the demo. The appropriate size seems to be 400 by 400 px.
Update the post. When you go back to your home page, you should see the right image being displayed along with the post title as a like to the post itself.