To set the co-ordinates you need to have the image in front of you and even if you use a simple program like MS Paint you will be able to see the X,Y coordinates change in the status bar when you move the mouse.
So that should give you most of the information you need obviously you can mockup/draw the hotspot first and the grab the coordinates from that.
You can have; rect | circle | poly | and COORDS=“left, top, right, bottom”. For rect: left-x, top-y, right-x, bottom-y; the circle, which specifies a circular region using COORDS=“centre-x, centre-y, radius”; poly, which specifies a polygonal region using COORDS=“x1, y1, x2, y2, …, xN, yN”. The first x and y coordinate pair and the last should be the same to close the polygon.
Coordinate values are relative to the top left corner of the object and may be expressed as pixels or percentages.
For example:
<map id="imagemap" name="imagemap">
<area shape="rect" coords="0,0,118,28" href="rectangle.htm" alt="Rectangle" title="I am a Rectangle" />
<area shape="circle" href="circle.htm" coords="184,200,60" alt="Circle" title="I am a Circle" />
<area shape="poly" coords="276,0,276,28,100,200,50,50,276,0" href="polygon.htm" alt="Polygon" title="I am a Polygon" />
</map>
<img usemap="#imagemap" border="1" height="280" width="280" src="map.png" alt="Image Maps" title="Navigation" />