Thanks, Michael! (And thanks for helping so many people around here.) ?? In case it helps anyone else:
I copied content.php and content-single.php to my child theme. I then moved the function “twentysixteen_post_thumbnail” (aka, the “Featured Image”) to inside the entry-content div, like this:
<div class="entry-content">
<?php twentysixteen_post_thumbnail(); ?>
The only negative side effect is that this pushes the text down a couple of lines. I fixed that with:
.home .entry-content > a {
margin-bottom: -3px;
}
.single .entry-content > div {
margin-bottom: -3px;
}