Many reasons to need to crop an image. Making rectangles square, or making squares into rectangles. Cutting a lot of useless background to bring out the subject better. etc. To get to the crop tool, you can either press the button on the toolbox or right click on the image and follow the menu Tools → Transform Tools → Crop in the image window. This will change the cursor and allow you to click and drag a rectangular shape. The button in the toolbox is the nicest way to get to any of the tools. I have chosen one of the huge and beautiful APOD images [APOD05].

I always click on the approximate upper left corner and drag to the lower right corner. You don't need to worry about being accurate on this first swipe with the crop tool.

After completing the click and drag motion, a little “Crop and Resize Information Dialog” (shown above also) pops up, telling you information about the borders that were defined in the click and drag. (In GIMP 2.4, the cropping tool has been changed. See Section 4.4, “Crop”.) We will have to change all of the numbers. If you would like to make this rectangular image square, you should find the width and height from the Get Image Information Quickie (see Section 4.5, “ Find Info About Your Image ”. Use the smallest of the two lengths to determine the size of the square. In my 300 x 225 pixel image, the largest square I can get is 225 x 225 pixels, and I will need to make sure the Y origin is 0. At that point, I use the image and the squares to get the best part of the image for the area. The upper right and lower left crop squares will move the marked area. The other two (upper left and lower right) will change the dimensions of the marked area, so be careful. I have a screenshot of this, right after I fixed the width and height and the Y origin, but before the final positioning. The arrows show the move points. I decided that the image looked the best with the X Origin at 42.