Click 
. The cursor changes to a crosshair.
Select an area by clicking and dragging the crosshair to draw a box around the area you want to view.
The Mask tool removes part of an image from view, enabling you to view only a section of the image.
To mask an image:
Click . The cursor changes to a crosshair.
Select an area by clicking and dragging the crosshair to draw a box around the area you want to view.
To maintain a portrait aspect ratio, press and hold the Shift key on your keyboard while using the mask tool. To maintain a landscape aspect ratio, press and hold the Ctrl key.
Release the mouse button. The image is masked to show only the area you selected.
To revert to the original image, click again.