This is my favorite border. You can make it expand to fit the image without interfering with the image and it makes a new layer so if you want to add a pattern or something to it. Add it to your GIMP scripts folder and refresh the scripts. It uses the foreground color (in this case, black). You can also use two different colors and make a gradient border. Cross script
You find it in Filters / Decor /
Apply Border. Be sure it's the Apply Border script, because there is more than one border script in there.
I've attached the script below: Add_Border_Script_by_Insanity_Prevails.scm