Sliding navigation bar

Hello all,

my page is here:

I’m about to embark on a navigation tool which takes the form of a sliding bar.
(top left of page)

What I’d like is to have it so that as the bar slides up and down the image to the left of it changes. Then when you release the button the corresponding page loads.

Any ideas what the code would look like for this sort of effect?

I’m not sure where to start, or if maybe I’m over complicating it and I should just go with a row of buttons or something?