There is already a plugin created by some one, called light box gallery. It is ok. But is not the neatest way to display more than 3 pictures, a tiled gallery? Because nodebb lavender theme uses the full screen area, so there is lot of real estate to display multiple pics in proportionate tiles rather than in list format.
This way of tiling will be a tidy way to display more than three pictures. Is it possible to include this by default in NBB? people who does not need this feature can simply disable it via admin panel, just like wordpress admins disable tiled galleries in the admin panel. Considering the fact that picture sharing is getting really popular nowadays, especially through smart phones and all, displaying the uploaded pics in a grid format would be a good addition to the UI.