Showing cursor position over an image

[toller]

I know this is not strictly to do with CMSMS but it's something i'm having a lot of trouble with.  The only example i've found of showing what i'm trying to do is at www.milliondollarhomepage.com. 

Before the main image on that site loads if you move your cursor over the grid it gives you your cursor position within the grid.  I'm trying to do something similar but can't for the life of me figure out how to do it.  I've found a few little javascript bits and bobs but they give you the mouse position on the actual screen and not within a specific image.

Anyone point me in the right direction?

thanks