Draw rectangle on imagemap

Hi there, 
Is there a possibility to draw a rectangle on existing imagemap on asp page?
Something like:
 
Graphics g1 = Graphics.FromImage(ImageMap1);
 g1.DrawRectangle(Pens.Green, 1, 1, 100, 50);
 g1.Dispose();
 
Obviously, there's an build error: Argument 1: cannot convert from System.Web.UI.WebControls.ImageMap' to 'System.Drawing.Image'
Any workaround? Thanks in advance