For anyone looking to edit a picture with one or more persons down to just a single person in the picture, you can use GIMP. This video tutorial goes over how to utilize the paths (or pen) tool and foreground select tool. The paths tool allows a person to select a part of an image to create a series of lines, or paths, that allow you to create a path around whatever you want to select in a picture. This item can then be moved and copied to a completely different photo. The foreground select tool is useful when you have a photo where the subject is distinctly separate from the background of the photo. You simply click on the tool and outline the subject of the photo to select and remove this image to paste it to another photo.
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