Alternately you could, upon making certain that your BG is set to black, go into Canvas size and with the box in the center  and set to pixels, adjust the size of your canvas up to how many pixels wide you want your border to be...eg: for an 800x600 image you would change the size to say 850x650 and hit ok and tickety boo y' have a black border 25 pixels wide around the image

 

Once in a while I look around,
I see a sound
and try to write it down
Sometimes they come out very soft
Tinkling light sound
The Sun comes up again