In order to (actively) select the area within the picture, move the mouse and point the location where you wish to begin the selection. Left-click on the image. Each new left-click while pointing a different area of the image will create a rib. In order to finish the selection (and close the polygon), either right-click the mouse on the image, left-click outside the image or double click on the image (figures no. 4 & 5)