Open your image, select it, and then go to "crop selected bitmap".
now this is where you have to pay attention.
When you move the mouse to one of the corners of the image, you could see a couple of different pointers.
You're looking for the one that's two diagonal arrows as opposed to the traditional "crop" pointer.
When you see the proper pointer, hold down shift to resize the selection and maintain the aspect ratio.
Once you have the proper crop size just press enter to complete the action.
