If you hover over your picture you will see that your cursor is now across. To select the area you want to crop you will start at one corner, left click once and hold down while you drag your cursor across the image. Once you have selected your crop area it will outline the area you chose. If you would like to move your crop lines hover in the middle & you can move it around so that it is aligned exactly where you would like it.

Once you are satisfied with your crop selection, double click & it will crop your image down to where you want it.

That's it!