From OCAU Wiki
Creating frames in Photoshop
To create a simple frame in photoshop for posting on the internet, follow these steps:
To create the frame it requires changing the canvas size
(To edit the canvas size, Image > Canvas Size or ALT+CTRL+C) Once in this screen, it's necessary to click Relative. Also change the new size from CM to pixels
For this photograph I have deceided to use a white frame with black around the image
To do this, I change the canvas extension colour to black and use 5px for both the width and height.Hit ok and there is now a small black frame around the image.
To complete the frame, reopen the edit canvas size window and now change the canvas extension colour to white and this time use 75px for both the width and height, hit ok and your done.
Which results in this being the finished product:
You can then add a title or your name to add a final finishing touch