I believe themes used to do it this way a long time ago, before “featured image” was actually implemented in wordpress. It would take the first image in a post and use that as the thumbnail when the article was shown in other spots, like the categories list. At least my old theme that was last updated a few years ago did it this way.
Most modern themes currently show the featured image as the first picture in your post. (So this would be the reverse of what you requested, but would have the same result.)
You can currently do this in the Hueman theme by selecting “image” as the post type when you are creating a new post, then add a featured image. It will be shown at the top of your post.
However I recently posted a topic about this, hoping that there is another solution, because it doesn’t seem to make sense to use an image post type in regular articles.
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This reply was modified 7 years, 7 months ago by daydull.