@lemoose That's only to sign in with your account for the first time, after you've done that you should be able to activate all the mods and upload normally / do what I said.

@edensk @ArcturusAerospace I tried Designer Suite and the game says I must disable mods to upload. perhaps this is only as of this latest update?

Open the image(s) you want to have as thumbnail(s) with Designer Suite, move it/them far away from the actual build, and take the pre-upload screenshots of them.

It's a mod.