Hi Karl,
in my opinion the selection of the preview image is a kind of "sub-optimal". In the gallery you have the image thumbnails where you see what you want as the preview image, but then you need to remind the image name, move over to the folder settings and select it from a dropdown which even has no search functionality. In larger galleries that's somehow annoying.
I'd love to have the function "set as preview" in the gallery context menu, that would make life much easier
Marco