Open the layers palette, undock the palette and move it out of the way. Add a new layer beneath background. Hold the control key and click on the new layer icon. The size of canvas can be increased.
Click on Image > Canvas Size. Enter new dimensions and resize the image. The border will be made with the foreground color. Choose any color and fill it with selected layer.
The background color can also be changed and the rectangle tool can be made around the image.
Click Window > Brushes palette. Modify the attributes of brush palette. Click on stroke path icon. The border will appear on image. Adjust the boldness and sharpness of the border area. The image can be made with the border of your specifications.