Click 
.
Position the cursor over the image and click at each corner of the polygon.
Finish the shape by double-clicking the end point of the line or shape. The line or shape is displayed on the image, and nodes mark each corner.
Enhancing Images > Using the Tool Box Toolbar > Using the Drawing Tools
Use the Polygon line tool to draw an irregular shape or line around an area of interest in an image.
To draw a polygon:
Click .
Position the cursor over the image and click at each corner of the polygon.
Finish the shape by double-clicking the end point of the line or shape. The line or shape is displayed on the image, and nodes mark each corner.
To change the properties of the line or shape, use the Annotations Properties toolbar. See Changing Annotation Properties for more information.