This theme basically uses the image if the post-specific hero is set, otherwise it uses the first letter of the title as the thumbnail.

But, you can set the thumbnail you want to expose in the post list, not the hero or the first letter of the title.

Unlike hero, this setting does not affect open graph or JSONLD.

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title: post with thumbnail
date: 2019-01-21 18:09:14
category: Front matter
hero: https://farm1.staticflickr.com/817/40620740124_9285c8b565_b.jpg
thumbnail: http://placehold.it/430x220&text=thumbnail