Still struggling with the image order, and noticing something else: If you sort your gallery (thumbnails) such that the first thumbnail is NOT the first image uploaded, AND you have the gallery images displaying as attachments, paging through your gallery gets weird.
Next & Previous do not take you through a circular list.
And since you start with an image somewhere in the middle of the list, you cannot view the whole gallery using Next, but will have to back up through the images already viewed to get to the ones you have not seen yet (and who will do that without knowing about the problem?).
Another feature I observed after inserting custom CSS into the approved section of Weaver II theme: using Previous to page through the Gallery, I suddenly found myself paging through all my (hidden) Header images… and could not get back to the original gallery images.