How do extra forum skins get added to the forums? Do they just have to have a specific layout, certain picture/background size, etc? Can we, if we know the format, submit forum skins?

You can't really make a layout unless you install your own forums somewhere, make it there, then export it. You can upload your own images, but you have to create a new layout and edit templates and such.

So yes, you can make your own skins that way.

Niiiice, because I still have the Lusternian Idea Development forum. I might do it there, see if I can export.

make sure its the same forum platform..

It is, it's the invision power board.