What squarespace calls a “thumbnail” is very likely to be the most important visual information on a given page. So I was looking for a way to replace the standard image with a gallery in the topmost spot of my Adirondack template based page but I there is no way to do this. My workaround is an animated gif but besides image quality there are just too many drawbacks to this approach.
So I suggest that squarespace updates the template(s) in a way that the thumbnail may be a container for a picture, a gallery or even a video clip. Would be a great thing!