'Cropping' means trimming the edges off a picture. This is such a basic and straightforward operation that you might be wondering why I did not include it in the previous lesson, 'Corrections'. This is because cropping is often deliberately used to change the impression given by a picture, and that is much more than just correcting faults within the picture. As an example, look at the picture below...
I think the picture would be better without the people, since it would give the impression that the elephants are wild (which they are not, for I took this picture in an elephant reserve in Phuket, Thailand).
To crop the picture, you can use the crop tool . Once the crop tool is selected, drag to create a selection rectangle that includes the elephants and as much water as possible but none of the people. If the rectangle that you have created is not quite right then you can use the mouse to pull the 'handles' in the corners of the rectangle to change its size or pull the centre of the selected area to change the position of the rectangle. When you are satisfied with the selection then press the Enter key or just click inside the cropping rectangle (or Escape if you made a mistake with the selection and wish to try again). Save (export!) your work before continuing.
A very common problem in photographs is that the photograph is slanted because the camera was not held exactly horizontal or vertical when the photograph was taken (see the example below, taken in Tahiti). In order to solve this problem the image must first be rotated and then cropped. In the GIMP, choose the rotate tool or Layer>Transform>Arbitrary Rotation or Tools>Transform Tools>Rotate or just Shift+R. First, set the options for this tool to include Clipping: crop to result and Transform direction: forward (normal) and then try rotating the image. It works OK, in an intuitive way, but there's a problem: it's hard to know when you have rotated the image the right amount. So press the Escape key to abandon what you were doing and try again with this new setting: Transform direction: corrective (backward). Now you have to twist in the opposite direction until the grid lines up with the horizon - a little counter-intuitive but ultimately better since it allows you to be sure you have the correct adjustment. To crop the rotated image choose Image>Auto-crop image, then save (export!) your work.