Use the left mouse button to outline the area (selection), click on the Edit Menu, then Crop selection.
The area outside the outline will be thrown away, only the inside will be left. The size of the image changes.
Use this to eliminate unwanted space (usually blank space) from an image. It can also center a badly composed photo.
Auto crop borders
This feature will remove borders/area of same color in the image.
The actions may be reversed with the Edit Menu’s Undo command.
Hint: for JPG images, you should better use the lossless crop in Options->JPG Lossless Crop menu.