You might have the "scrape missing link previews" setting enabled in Thunder. It's in general settings.
Instances will provide a preview thumbnail for links, but will sometimes fail to find one for a given link.
Thunder is able to have a second go at finding an image to display with a link, when the instance doesn't provide one with a given link.
There is a setting for displaying a text preview of the post body, in settings>appearance>posts.
You can also just search the settings to find these.