This allows you to choose part of a picture (or part of PDF/document
page) to display, rather than using all of the original image.
Start by putting a tick in the 'Manually crop picture' tick box and then
use the 'Crop image full screen' button to see are large version of the
picture which can be graphically cropped.
If you look at the above image, the part of the image selected with the
green rectangle is show normally. All the other part of the image,
that will not be shown, is pinked out so that it is easier to see what
you are getting.
YouTube videos
This video shows how to crop with the picture control, which is exactly
the same process:
How to manually crop an a picture in Repeat Signage