Why is it that just about every photo has stuff you don't want in it, like too much sky, too much ground, or the main image is off centered.  Cropping will take care of some of these problems for you.  With the image loaded, you are now ready to crop the photo.

Like most options in The Gimp you can get to the crop tool by two different methods. You can pick the crop tool from the main window:

Or you can right click on your image like this:

Now just move the guidelines around until you have what you want.

When the lines are where you want them, then double click and you now have the picture cropped.

Now lets optimize it for the web.