Hey dougal,
Thanks for the compliment, I commissioned the Light Box hack from Mark way back last year and was pleased he published it to the VM community. I'm glad to see that it is very popular.
You need to do a couple of things.
1. upload a few images against a product using File Manager.
2. On your flypage place the {more_images} tag at an appropriate place
3. Now the tricky bit. In shop.product_details.php edit the /* MORE IMAGES ??? */ bit - I have attached my edited 'More_images' code. PLEASE NOTE: I HAVE NOT UPDATED MY HACK TO THE LATEST SO YOU WILL NEED TO BE CAREFUL.
Essentially all I have done is wrap the images in a div, changed the Javascript event handler to 'onMouseOver' then apply a css style to that div
border:1px solid #000000;
float:left;
height:20px;
width:20px;
margin:2px;
overflow:hidden;
what this does is creates a small 20px x 20px box with most of the image hidden because I have used overflow: hidden.
I must say that I haven't tested in all browsers but it does work in FF AND IE6/7.
PLEASE BACK UP BEFORE YOU EDIT.
good luck