How to make part of an image an href?

Question

I have made a website, and would like to make the subscribe button clickable (an href).

So far here is my code for that. My problem is that I don't know how to do the coordinates. Is there a website I can use?

<map name="subscribe"
<area shape="rect" coords="0,0,82,126" href="http://www.google.com" alt="Test">
</map>

Here is my image:

Answer 1

Map Area coords are: left, top, width, height

http://jsfiddle.net/5sXAb/

<img width="600px" src="https://i.sstatic.net/AQ4BD.png" usemap="#subscribe">

<map name="subscribe">
<area shape="rect" coords="230,55,385,100" href="http://www.google.com" alt="Test">
</map>

Ninja edit; there are ways round it, for example using imaging software to find the coords, a javascript code the adds a border, or my preferred option - using transparent divs instead.