I've visited some websites which displays a small image but when you click on it the photo enlarges and the rest of the page greys out, is this javascript which does this and is it simple to do?

Google search for "Lightbox"

What happens is when u click they creating a div on the top of everything basically to cover all the client area, I personally insert PNG image into the div 100%x100% with opacity what ever u need, it will cover all the select boxes in IE6.