Let's say that you're giving the image a border of 50 pixels each side. Your background layer will be 100 pixels bigger each way. Put the image layer in the exact top left corner of the background. Select the 'Move' tool. Hold down the shift key and press the 'right arrow' key five times. Still holding down 'shift', press the 'down arrow' key five times. Ta-da! Holding down the shift key means that the arrow keys will move whatever is selected by 10 pixels. If nothing is selected, it moves the whole layer.