I'm not sure about Elements but I would assume it would be similar to Photoshop.
Anyways, in Photoshop you can use the the Rectangular Marquee Tool (the dotted box that you use to select/crop with) to select where the border will go. Then you can go to Edit > Stroke that will bring up a dialog box. Select the width of the line, the color, and opacity (if you want it slightly faded) and click ok! You should now have the border inside the image.
You could use the above to also do an image that juts outside of the border too. Create a new layer before you do the above steps then after the border has been put in you can use the eraser tool to delete the lines that cross the original image then flatten the layer. Of course this wouldnt be considered DPC legal, but it can be a nice effect.
[1] Create a new layer on top of the image and select it in the layers palette
[2] Choose Select | All
[3] Choose Select | Transform Selection
[4] In the bar below the menu operate in percentage and choose something like 96% for both witdh and height (change 96% if you want the border bigger or smaller).
[5] Choose Select | Modify | Border with something like 1 pixels (change 2 if you want it thicker or thinner).
[6] Choose the Paint Bucket Tool and fill the border with your color of choice
Now that you happily ended with the border you liked, you can submit your photo to DPC and have it DQed because you used layers and spot editing illegally! =)
BTW: there is a rule which allows borders and I suppose that it overrules the limitation for spot and layer editing.
