Moving a gallery
To re-order galleries, drag-and-drop their cover thumbnails to their new position (My Images & Videos panel).
Galleries can be nested within each-other without limitation. To move a gallery under a new "parent gallery", drag-drop its cover thumbnail to their new parent in the left-hand side column.
Alternatively, open the Name and main settings dialog for that gallery, and type the name of the new "parent gallery" under the Goes under… section.