Unsure what makes the facebook display of two of my articles different

There was no attachment in your post. After you upload the images, please make sure you click the green arrow to include them in the post.

With that said, the way that Facebook chooses the image is based on the dimensions of the image that are set for the page. Wider, higher resolution images will get a larger format in the posts and be displayed above the summary section. Smaller and/or lower resolution (<200x200px) will be shown in a square on the left.

There's no way to force Facebook to use a specific image format as there's a huge variety of places that the feed post can show. If it's on a Facebook Page, for instance, you can choose to 'feature' the post, which will make it take up more space and usually will try to show a larger format image.

So, bottom line, if you want the square image, use lower quality images.. which seems a little odd, but makes sense for the lesser prominence that those images are given in feeds.

I hope that helps,
Alex