ImageSize function

  DECLARATION:  ImageSize(x1, x2, y1, y2 : Integer)

      PURPOSE:  Returns the number of bytes required to store a
                rectangular portion of the screen.

         UNIT:  Graph

  RESULT TYPE:  Word

      REMARKS:  x1, x2, y1, and y2 define a rectangular portion of the
                screen.  ImageSize will determine the number of bytes
                necessary for GetImage to save the image.  The number of
                bytes includes space for 2 word-type variables that hold
                the width and height of the rectangle.

 RESTRICTIONS:  Must first be in graphics mode.