Something I learned by accident because G+ apparently wants its interface to be a source of surprises: when you post a link from a web page with multiple images as here, before you publish the post you can select the preview image displayed with the post.  When you hover the cursor over the image, little forward and back buttons appear allowing you to cycle through whatever G+ recognizes as an available image.  This option isn’t available after the G+ post is published, however, so you can’t go back and edit something that already has the wrong image.  Just one more way G+ posting options are obscure and puzzling…