but the Blog functionality in Shopify is limited to a strange little column
It probably seems that way as you have only one blog post.
Image may be NSFW.
Clik here to view.
If you are wanting a WordPress style featured image, it would be easy enough to find the first image within article.excerpt or article.content, and then used however you need. Even if there was an app, it would be overkill in this case.
