Sorry Rolf, ignore this question – it is already working fine (on mouseover = pause).
I thought it wasn’t working because the mouse was over the lightbox, but the slideshow was still running. I realise now the mouse not only has to be over, but it also has to move in order for the pause be invoked.
Actually I just noticed it requires the mouse pointer to come from outside the picture, cross over the border, and arrive over the picture element. In other words, a ‘mouseover’ event.
Personally I think users would figure it out quicker, if the slideshow paused upon any movement of the mouse while it was over the picture. In other words a ‘mousemove’ event. (Related SO question)