I have used the Stellaris launcher to add a mod to the Steam workshop for the game of the same name, but after doing so, it appears with a blank thumbnail:
Going from the steam side of things, you can manually upload an image, but there doesn't seem to be an option to select it as the thumbnail:
I can't for the life of me seem to find anywhere to actually set the thumbnail. The in-game upload screen seems to be lacking this as well.
Is this something doable in-game, or is there some workshop setting that I'm missing? (There definitely are mods for this game with thumbnails)