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3Welcome Hiền Anh Lê! Thanks for posting your question. Unfortunately, GD.SE is not designed to handle issues regarding basic software education. If you are seeking to learn how to accomplish something within an application, please review the application help files or try an internet search with your question or an internet search for a tutorial. We are confident you will find answers. If you have questions about GD.SE, have a look at the help center or feel free to join us in Graphic Design Chat once your reputation reaches 20. Keep contributing and enjoy the site! – Vincent Oct 27 '16 at 11:54
This is a basic task in most image manipulation applications - for GIMP, you can use the crop tool.