Can you add blank space to a photo in Mac Preview?
A simplified method that works just as well:


Open your picture in Preview.
⌘ command + A and then ⌘ command + C to copy it to the clipboard.
Go to Tools &gt; Adjust Size... and enlarge the image to the overall size you want for the canvas. (I would make it bigger than you need as it is easy to crop at the end.)
⌘ command + A and then delete to remove the enlarged image
⌘ command + V to paste back the original, smaller image onto the enlarged canvas.
Choose Convert when asked to "Convert this document to PNG" (as you can always convert back to JPG later).