How to make a forum like this?

[Uranium-235]

as far as I know, it has no limit, but if theirs like thousands of images it might take a while to load the way it works (loads the directory into an array, and selects a random number in the array varable $images[<random number>];

the loading into the array might tke a while if theirs lots of images.

The script will require a server that has PHP installed. And you will have to know how to operate it, if you have a server with php installed, I can post the source code here.