The key is to use the “Intelligent Scissors” selection tool. The keyboard shortcut is “i”. Add the control points by left clicking the edge of the area you want to select, make sure you select an closed area by finally clicking the first control point. Then click the inside of the selected area. Now the selection is finished.
Now you can use “Ctrl + c” to copy the selected area. Then click menu “File -> New…” to create a new file, then use “Ctrl + v” to paste the area, save the file to create a new photo.