The actual image mapping part is simple, but extremely tedious. So, the easiest way to do it is to use an image mapping tool. (ex. http://www.image-maps.com/).
After you finish the mapping there are parts in your code that look like this:
<area shape="rect" coords="10,10,60,60" href="url" alt="" title="" />
Pretty much what this says is the area that you are mapping, in this case a rectangle, has the specified coordinates. Next it tells you were that area that you mapped is linked to and finally the title which will display when you mouse-over the area.
From here, with this website, comes the HORRIFICALLY painful second step, if you do not write it down ahead of time.
Here is an example of one in my story:
<area shape="rect" coords="407,460,539,586" onclick="PostBack('FollowLink','6');return false;" href="#" alt="Room 1" title="Room 1" />
Because you do not link to a specific URL you will have to link to a link. With this website there is no way to identify links other than jotting them down. Every time you create a new link in your story, jot down which number it is. Then where it says: ('FollowLink',6'); you want to input the number of which link you want to go to.
For example, if you have an option of clicking on an orange or an apple and the link that you want the orange option to be on is the 7th link you created and the link that you want the apple option to be on is the 14th link you created then input those numbers in the above code.
Hope this makes sense, I am bad at explaining things. If you have a question just post a reply and I will try to explain it better.