I would love to see a thumbnail decoration for any thumbnails that are included in a gallery. Currently the only way to check is to click properties (alt+enter) or visit the gallery. I think it would be a useful decoration to have and would give users an immediate visual indicator that a thumbnail is linked to at least one gallery, then I can click properties and find out which galleries the image belongs to.
I have submitted this request to Cerious (almost a year ago) [Cerious Case 1233781] but it seems to have stalled.
I imagine a gallery thumbnail decoration would be quite simple to code but probably won't get Phillip's attention if no other users are asking for it, so please add to this thread if you think a gallery decoration would be useful.
