If you have set your article layout to the Two-column View, then your Authory page will look something like this, with each article displaying a small thumbnail image:
The One-column and List View also show a thumbnail. You might want to change or, if there is none, add a thumbnail image for an article. Luckily, that's rather simple if you know how Authory chooses thumbnail images:
By default, Authory will always display the top picture that you see in the article's full text view as the thumbnail image.
If there is no image in the article full text view, then Authory will try and use the image that's been linked as part of the meta tags on the respective article's original page.
If there is no image in either the article full text or in the meta data, then Authory will show a placeholder.
So, in order to add or change a thumbnail image, the only thing you have to do is add or replace the top image in your article full text view. And we have a handy little guide for how to change/add images.