I’m not too sure how to apply a border to one side of an image using Photoshop CS2, however, if you do wish to add a border to an image, select your image layer, go to blending options > stroke, and apply a border.
EDIT>STROKE to add a line around all sides of the image. You can specify the colour and the number of pixels wide the border will be.
To add a wider border (like a matt) go IMAGE>CANVAS_SIZE. This dialogue also affords you the flexibility to apply the canvas extension to one, two, three or all four sides.
You can also add a stroke as a layer style…make sure you select "inside" if it’s going all around the entire image or it will end up outside of the image area. (Same goes for Chris’s first suggestion.)