How do I add a thumbnail image to my Stellaris mod on the Steam Workshop?

I'm sure by now you solved this, but I'll leave here my answer for others that may encounter this issue. I had this issue too with my mods https://steamcommunity.com/id/savalina/myworkshopfiles/ and didn't find any help elsewhere, but I sorted it out.

Make sure you add the thumbnail tag picture="picname.png" in the yourmod.mod file, not in the descriptor.mod file located in your mod folder.