Kind of figured this out, well a work around anyway. I think it is a bug in the forum code and not really a B&C issue. If you load the forum from a post the bug doesn't happen, even if you navigate to the front page. If you load the frontpage it happens every time, unless you click a link before the pages finishes loading. I noticed when I was using my iPad via cellular data instead of wifi, (a little slower) the page loaded completely and correctly before adding the faulty images in at the last tick of the loading bar. It could be a skin issue, or it's just a issue in the code itself, and how it interacts with the iPad version of Safari.
Even if I let the images load, if I clear my cache by closing the app, then restarting and immediately clicking any link on the page, the images don't load again and I can use the forum until my cache clears. (IPad off too long, or using another app too long.)
So long as I load any page other than the front page fully, first - the forum works correctly, every time.