To take a picture of part of your screen:

Choose File > Grab > Selection.

Move the pointer to a corner of the area you want to capture.

Drag the pointer across the area and release the mouse button.
You can take a picture of any part of your screen so you can save or print it.
Choose File > Grab > Selection.
Move the pointer to a corner of the area you want to capture.
Drag the pointer across the area and release the mouse button.
Preview captures your selection within a few seconds. The image is opened in Preview and saved to your desktop.