From Shalane's Personal Site.
Image maps are one of the surpringly simplest things to do. I will show you how to do it in two ways:
Guess and check and Using Paint. Let's do the paint method first as it is the easiest.
001. First things first, have an image you want to make into an image map. I am going to use this lovely little one I made just for the tutorial. I am planning to make the numbers all linkable.
002. Now open up two programs: wordpad (or your text editor) and MS Paint. Copy the code in the textbox below onto the wordpad and open the image you are using for the image map into paint.
003. Well now is the time for me to explain a little bit about image map codes. The code is pretty basic. Let's look at the same code you should have open in wordpad.
The shape="rect" stands for the shap being a rectangle. The coords are the coordinates for the rectangle on the image. And obviously the href is the link it is going to. So the coordinatees are the tricky part. So that is why we are going to be using paint for a little bit of help.
004. Okay. So you have your paint window open, right? Okay. Now, select the rectangle selection tool. Now hover your mouse over the top left-hand corner of the first "rectangle" we will be linking.
Now while you are hovering look at the bottom right-hand corner. You will see two number seperated by a comma.
Now those are our first two coordinates for number 1!
Now we need to hover over the bottom right corner and we will see two new numbers. I got 50, 50. let's Plug that in.
005. Now repeat the last step for each of the corners of each of the parts you want to have linkable. So after I do all for numbers this is my code.
006. Let's now plug in the other information, such as link urls and the image url. You can do this before we get the coords also if you would like.
And when we put it into action this is what we have:
THE OTHER METHOD
You can also guess and check using estimates. Each point has the two coordinates. The first one is how far left it is and the second is how far down (known as top). So just estimate and check. 
I hope you have made an image map with my tutorial and found it wasy to follow!