An easy way I can think of is to use a WYSIWYG
HTML editor like FrontPage. Open your HTML file, select the picture by clicking on it, and choose from Rectangular Hotspot, Circular Hotspot, or Polygonal Hotspot
at the bottom of the window, move your mouse back to the picture and you'll see your pointer turns into a pencil for you to draw the hotspot, after drawing the hotspot it promts you for the URL you want that hotspot to link to.
If the shape or size of the hotspot is not to your liking because it's very hard to create an accurate shape given the limitations of your mouse, then simply go into the source code and manually enter the coordinates of the hotspot yourself.
BTW, the link above that ExCyber
gave is good reference.