Several things learned, when trying to figure out how to get a nice image on articles shared to Facebook and Twitter.
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Sharing an article

I discovered that it is better to always embed a small thumbnail image onto the beginning of the article. Otherwise there becomes some arbitrary "widescreen_example.png" image retrieved from the s.ipernity.com, where also ipernity icons and JavaScripts are stored.

UPDATE: It is better to put the image somewhere around second paragraph. Anyway there should be at least one line of text before the image. It turned out the DeepL translations does not work for the article at ipernity if the image is the very first item on the article.

Use an image that either advertise ipernity, or describes the article topic. The ideal size for the original image used as a thumbnail should be at least 600 pixels wide and 314 pixels high. Any smaller, and it will not become displayed on the share. Facebook and Twitter seems to crop from top and bottom.


Twitter peculiarity

At some point the share button for Twitter has stopped to work. So, to share at Twitter, one has to copy the article page URL, paste it onto a Tweet, and write a few words or add hashtags before the URL. Twitter then retrieves similar image and link as seen at Facebook shares. One may have to wait a little before the embed image becomes created. So, be patient.