I just noticed something interesting.
This forum posts seems to use ~950px wide images, when its full sized. Anything above that gets a Zoom option, and anything below that is displayed at its original resolution.
Since the "Miniature BBCodes" on my last post are 180x180px each, I could display 5 of them side by side.
Also
@confusis , you say "there's no inbuilt image sizing/placing on the forum", but there actually is! See an example:
When you quote a full-sized image, the image gets resized automatically, to a determinate smaller px width.
So, maybe with a bit of coding it is possible to add this feature to threads?
For example, you can create the options for 2x2 and 3x3 pic grids. If I'm correct about the 950px post width, the 2x2 grid option would resize the pics to ~420px width, and the 3x3 to ~315px.
The side-by-side placing would be done automatically aparently, as demontrated on my last post.
Anyway, just throwing some ideas