You use a program to do an image map and trace a general guide around what you want. Dreamweaver has this feature, and surley others (GoLive, etc.) do. Do a search for image map on version tracker to see about freeware programs.
one problem i have noticed is that polygon links in Dreamweaver won't run javascript like to open a popup window...or at least the popupwindow that i made
Even very basic programs like the old Adobe PageMill do them - you just have to double click on the image and you get a little set of drawing tools up to make the polygons with.