How do you make a picture Wider in GIMP? I don't want to stretch it I just want to take one side that is....?

all black and make it wider as to fit my desktop better. I've tried making the canvas size bigger but cannot figure out how to fill the spot on the canvas that has that grey checkerboard canvas thing with the Black from the picture. I have tried rectangle selecting from the image and moving it over but the selection is invisible. I tried the bucket fill tool but i am just not able to get anything to work in the area that i have expanded the canvas. Please help