I've installed lightbox into my shop which is working but it is only showing 1 large image for each product and for some of my products I have 3 large images. Anyone have any ideas as to why this would be the case?

My image naming is as follows:
main product listing image: images/z743.jpg
large version of product image: images/large/z743_LRG.jpg
alternative large image: images/large/z743_01_LRG.jpg

Any help much appreciated.

Thanks in advance.