When viewing images in a post that break out into the lightbox there is, probably, a Z-Index problem with the likes floating above the lightbox image (see below):

 

It may also be useful if the page scroll could be locked when the lightbox is active.

 

You should submit as a bug.  You should also test on the default theme first.  

The problem is showing on this board which I presume is the default theme (which is really nice BTW).  I will do as you suggest and submit as a bug - thanks for the reply.

 

My theme on this board doesn't look anything like your screenshot.

I reported it as a bug a couple of weeks back. It's apparently fixed.