1) I put the 2 urls in an array - which you can edit to suit if the urls ever change in the future.
2) I created a global variable urlIndex storing an integer which represents the indexes in the array in 1)
3) added an onmouseover to the <img> which sets urlIndex to 1 when the mouse passes over the image.
4) when the "Next" button is clicked it fires the go2url() function
5) go2url() redirects the current page to the url in the array in 1) whose index is the current value of urlIndex
You could do it with an image map and it will work but since in your case the coords cover the whole image, the image map is totally redundant and does nothing but add extra code to your html. Normally you would not use an image map for what you are tring to do.