I had searched web how-to's on using gimp to do this. But after spending half an hour to forty-five minutes trying to do in gimp, had no success, and felt no smarter about gimp, so gave up.
Today I stumbled across a bit map editor http://www.linuxinsider.com/story/75531.html that runs on Windoze/Mac/Linux and
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$ apt-cache search pinta
So I installed it. APT told me the program occupied 4.7 MB of disk space.
So I opened the program, found a jpeg, and tried to crop the image, with NO previous experience and with NO additional web reading. It took me 7 minutes of experimentation, and the image was cropped the way I intended to. I wrote it to another jpeg file, and verified it was so using gwenview. Because pinta has unlimited un-do, I was also able to get the original image back by pressing Ctrl-Z about five dozen times.
What a pleasant surprise!