Thank you so much for your reply - you have a very quick eye for detail.
The reason why I didn't go with the second method is that it does not allow to put hotspots on each region (Americas, Asia, Europe). I need some sort of way of combining the two methods (i.e. rolling over two images synchronously). I put the third pair in the function when I was playing around with it - but it does not seem to effect the function when using 2 pairs.
You are absolutely correct that I need only two rollovers, so my difficulty is not with passing a third pair, but with getting OnMouseOver to perform two functions at once. Or could there be a different, and more elegant solution?
My knowledge in programming is very basic - please excuse.