Javascript & Gallery Help (easy for you guys)

Hi Guys,

I've been progressing along ok on the CSS side of things but am having some trouble on the javascript side of things. I've decided to jazz a site up with a scrolling gallery from a template i'm using but unfortunately I don't know how to get the main image to change on a click of the smaller one? can anybody please help a brother out!

here's the site template - http://dev.jeffking.co.uk/index.html

All i'm after is the big main image to change to the same (larger) image of the smaller one in the scrollbar below the main frame.

I'm pretty sure I will need to tweak the JS to enable me to do this but i don't know how or what to do. please can someone help me!!

Many thanks!