I managed to define the image to be queried for the thumbnail (thanks to the forum) on my panel blueprints. But I don’t know how to do it for my panel homepage (site yml I guess). Its picking up an editorial page of the site, but not the cover image.
I found this post, which seems to go into a similar direction, but it didn’t help me to sove the issue.
If you don’t define anything in your pages sections, the first image in the page folder is used, or if that doesn’t exist, an icon if defined in the page blueprint, or if no icon is defined, a standard icon.
The same logic is true if you define an image like page.cover.toFile but that image doesn’t exist, either because no cover image is selected in a cover field, or the selected image doesn’t exist anymore or because the field doesn’t exist.
The first view you usually see when you open the Panel is the site view, which is defined through site.yml. The homepage is actually a page with its own blueprint.