Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 91

For cropping images, the Crop Tool is available in the Toolbox. In general,
you can resize your image to any size you wish or crop the image any way you
want. Simply click with the tool into the image. Select an imagined point at
the top-left of the image and drag the cursor to the bottom-right corner point
by holding the left mouse button. This way, a rectangle with a solid border is
drawn. At first, the aspect ratio is not relevant. You can drag the rectangle into
any form you like. You can use the rulers on the side and top of the window
as a guide.
Select the Fixed option by selecting the check box. Initially, you can
find the current aspect ratio of the image in the box below the Fixed option,
measured in pixels. Overwrite this value with the desired aspect ratio of your
photo format (in figure 2.26 , it's 5:7). If you then draw your rectangle in the
image, it will always have this aspect ratio. The advantage is that this rectangle
can be drawn up to any size within your image. You can also click in the
rectangle and drag it into any position. This lets you very precisely choose the
image section you want to keep. Double-click inside your selection to crop
the image.
If you want to use the Crop Tool without a fixed aspect ratio, deselect the
Fixed setting.
This method is very fast when you want to crop several pictures one
after another using the same aspect ratio. However, image size has to be
recalculated (again) with the Image > Scale Image or Image > Print Size
function because pixels are being cut out and the image size changes. In fact,
it's recommended that you use the Crop tool first to set the aspect ratio and
to determine the area within the image you want to keep and then recalculate
the size and resolution. However, if the image section you select is too small
and the image is enlarged, the image quality suffers. The image may appear
blurred and pixelated, depending on the method used for recalculating.

• NOTE
If you want to remove the
selection rectangle, simply press
the Esc key on your keyboard.