Click 
.
Position the cursor over the image and click and drag to draw a line. The line is displayed on the image, and nodes mark each end.
Enhancing Images > Using the Tool Box Toolbar > Using the Drawing Tools
Use the Line tool to draw a line to emphasize an area of interest in an image.
To draw a line:
Click .
Position the cursor over the image and click and drag to draw a line. The line is displayed on the image, and nodes mark each end.
To change the properties of the line, use the Annotations Properties toolbar. See Changing Annotation Properties for more information.