Tim
A couple of quick ways –
Layer>duplicate layer
CMD+A – select all
Edit>stroke
Pick your width, pick your colour, select inside, OK
(This covers the outside edge of your picture with the border)
or
Set colours to default – D key
Swap – X key
Image>resize>canvas size
Input height/width increase (in pixels)
OK
Possibly other ways that I’m sure others will add.
Paul
Here is an idea you might like to try. You could increase the size of your canvas, and this would immediately put a white border around your picture. For instance, if your picture is 4×6 and you change the canvas size to 5×7, you would now have a one-half inch white border around your picture. Next, use the magic wand to select that white border. I like to copy that selection and then paste it back so that it is on its own layer. Now, just choose your color and use the paint bucket on the border layer. There are a lot of other effects that you can add to give your picture more of a framed look. You can use patterns and blending options on that layer to create some attractive effects.
Jim, if you make black the background color in your color picker before you increase the canvas size, the increased area will be black instead of white. One less step…
In looking back, I see that Paul suggested this approach too.