This mod permits you to orient thumbnails horizontally within an attachment window, so that you can achieve a gallery effect:
http://www.phpbb.com/community/viewtopi ... #p12869215
Nice! However - unlike when you place attachments inline, you can't control the placement of thumbs after the fact in an attachment window. Furthermore, you have to attach them in reverse order! After which, if I want to insert somewhere in the middle, I'm out of luck.
As you add attachments, the entire list of attachments is represented. It would be great if you could move attachments up or down in this list, much as you can reorganize forums in the ACP.