1. So, you've got your picture. And you want a black border around it. But how do you do it? Simply create a new transparant layer (CTR-SHIFT-N) and call it "border". Select all (CTRL-A) and go to "Edit" and click "Stroke..."



2. A window pops up. Tweak the settings to your liking. "Width" is the thickness of the border and "Color" is the... yeah... color. The "Location" of the border is important to define. In most cases you will have to go for the "Inside" option. This will lay the border inside the selected area. If you for example chose "Outside", you wouldn't see a border, since it would create the border outside the canvas. That's it. It isn't that difficult. But it could sure be easier to figure out on your own!