If you frequent image-heavy websites such as Pinterest or Tumblr, you've likely already seen a masonry layout. Unlike an evenly-packed grid, columns of images (or other items) are allowed to vertically cascade and space themselves based on available image height; much like how a mason would arrange a wall of bricks.
As someone who is investigating XF as a solution for flexible, thread-based image galleries, I think this would look great displayed inline within posts. Including this in the XF/XF2 core as a global setting (or optional post flag?) would be an appealing visual addition on top of the existing grid layout that would also lend a bit more of that 'modern' aesthetic to XF-powered forums.
As far as development is concerned, there exists low-footprint, fast-performing JS libraries -- such as 'bricks.js' -- that may be of some help.