I want to create a javascript that when you click on a thumbnail of an image the larger version of it pops up in the center of the screen.

I've done this (mostly) however I have a slight problem, it centers it fine, however it is only centered when you are on the top of the screen. Some how, I want it to be offsetted so its centered at the part of the site you are looking at. (and not have it follow the screen as you move up or down)

You can see what I have done here (link: http://www.jonsully.co.uk/ ) by clicking on a blog image thumbnail or one of the first two in the portfolio section.

Anyone have any idea's of how I can offset the image so its at the center of the screen where you are but not following it?

(btw, don't try it in IE, it doesn't center atm, I will get round to making it work in IE later or sth)

Note: Now works in IE