Click 
.
Position the cursor over the image and click and drag diagonally to draw an ellipse. The ellipse is displayed on the image, and nodes mark the outer corners of the shape.
Enhancing Images > Using the Tool Box Toolbar > Using the Drawing Tools
Use the Ellipse tool to draw an ellipse or circle to emphasize an area of interest in an image.
To draw an ellipse:
Click .
Position the cursor over the image and click and drag diagonally to draw an ellipse. The ellipse is displayed on the image, and nodes mark the outer corners of the shape.
To change the properties of the ellipse, use the Annotations Properties toolbar. See Changing Annotation Properties for more information.