Quick Borders
This is a handy technique to add borders to your images quickly and accurately, with plenty of flexibility and scope for creativity.
Open your image in Photoshop, reset your colour palette (Ctrl+D), Select the rectangle shape tool (Shift + U until the rectangle tool appears). Draw a rectangle of any size in the middle of your image.
Set the Rectangle fill to zero, double click the layer to bring up the layer styles, or right click on the layer and choose 'Blending Options' to access the same window. Click on Stroke to bring up the Stroke Options, set the Position to 'Inside' (Important!), adjust the Size slider to taste.
Press Ctrl+T to transform the rectangle, reposition and resize to fill the whole image, press enter to finish (or Esc to cancel).