Web page design help!?

If I make a grid in say excel, 1000 by 1000 squares and I upload the image of it to my website, how would I get it so show the coordinates of each square when someone scrolls over it? This is very similar to those ad space websites. When you scroll over a box on their grid it shows you what numbered box it is right next to your mouse pointer. HOW DO I DO THAT?!?! Thank you