A cluttered work area can be a bit distracting. By simply cropping the image you can focus in on what you want to show.
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In Gimp, click on the Crop Tool
(or
in other photo editing programs)
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Draw a rectangle around the area you want to keep.
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Double click within the rectangle and the area outside of the image is deleted.
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Save
For Instructables' step-by-step or photos square images work better. This is particularly important for the Intro photo, if they are not square part of the image will appear cut off in the thumbnail.