The following tutorial will show you how to create a PSD document pre-prepared with a frame layer and a background layer. This tutorial was created using Photoshop CS3, but should work equally well for most other versions.

1) Create a new document the size you want the final image to be (6x4, 5x7, 10x8, etc.)
File -> New
Change Width to 10 Inches
Change Height to 8 Inches
Set resolution to 250 DPI (or whatever you work with)
2) Select the entire document by hitting Select -> All
3) Go to Select -> Modify -> Contract
4) Set the border to the size you want (at 250 dpi, a 1cm border would be roughly 100 pixels)
5) Invert your selection so that you are only editing the border. Do this by going to Select -> Inverse
6) Use the Paint Bucket tool to paint the now-selected border black (or any other colour you want)
7) Without changing your selection, cut the border out; Edit -> Cut
8) Paste the border back in; Edit -> Paste
-------At this point, you have a blank "stock canvas" you can readily add an image to. I'd recommend saving this somewhere as a PSD master document you can re-use any time you want to add a border to a picture--------

9) Open your photo that you want to add a border to, then copy and paste the whole picture to your border document.
10) Ensure the photo layer sits between the background layer and the border layer
11) Resize\move the picture behind the border until your happy
12) Done