Group Head documentation is invalid
usermanual says: You can define which image constitutes the group head, while in an expanded view of a group, clicking on the “G” symbol of the desired image.
You can try these steps but it does not work. I suggest the following change:
You can define which image constitutes the group head, while in an collapsed view and clicking on the G icon the group expands with all
images having a lightgrey G. Clicking the G symbol of an image makes it the group head.