Hi Gerhard,
I can’t seem to visit the site you linked. Is it viewable at the moment?
I tested the site on a site availability checker, and it says that your site is currently down.
Can you check?
In any case, if its an archive Page, you can use a Block Element – Content Template, and then use a GenerateBlocks Image block to add the “thumbnail”. Doing so allows you to set a particular size for the image.
Reference: https://docs.generatepress.com/article/block-element-content-template/