How to create a custome scene in together mode?

I can not figure out how to publish/upload/preview a custom scene in together mode

I go to this place: https://dev.teams.microsoft.com

I pick one of the defaults scenes, like the Halloween one.

I then want to try it out. So i press the preview button

But then i get this error

Unable to find the scenes image file ........ in provided archive.

I dont understand this error.

Where should the images be located?

How do i upload the scene in teams?