1. Open an image in Adobe Photoshop.

2. Click on Layer- New Layer and press OK.
3. Press Control A to select all.
4. Click on Edit- Stroke and change it to 1px, keep it black (or you can change it to any color), and the location to inside. Press OK. Press Control D to deselect.
5. OPTIONAL If you want a white border inside the black border, press Edit- Stroke, change the color to white, change the location to outside, then click OK.

This looks better with a colored background.