For the record: Discourse Core has the ability to do this since February 2021. We may look at making it more discoverable in future, but for now you can manually edit the markdown of the image and set one as the thumbnail.
By default Discourse will use the first image in the OP of the topic. If you would like to select a different image from the OP, add |thumbnail to the markdown. For example
 << this one will be the thumbnail
(this was originally discussed over in the official Topic List Thumbnails topic, but this core thumbnail selection feature isn’t specific to that theme component)