I used to use the Gimp, but it is entirely unintuitive and overly complicated. I hate the GIMP. It promises so much, but delivers so little.
I type "shotwell xyz.png" on the command line and there is a crop button at the bottom of the screen. Using it is intuitive, then use "save" to save the cropped file in place of the original and you are done.
Here is the link that first led me to it:
Tom's Digital Photography Info / [email protected]