Yea, it probably could be best solved through cooperation but the thing that springs to mind for me is that card responses for link unfurling have a standard way of presenting an image, or text, or an image embedded in text - but there's no standard (even an unofficial standard) way to clearly respond that the link unfurling should be multiple images in an arbitrary gallery ordering. If multiple images are returned in a card the context reads like a blob of text with multiple images in series.