Old photos were printed in a darkroom, usually using a printing frame to hold the printing paper flat and ensure sharp edges. The side-effect of this is that old prints always have a white border around them, so we need to add this to our picture if we want it to look authentic.
There are several ways to add a border to a picture, but here's a fairly easy one that works in almost any editing program that supports layers. First, create a new Fill layer. In Elements you can do this via the Layer menu.
Set the solid colour picker to plain white.
Next, you need to re-arrange the layers so that the white fill layer is underneath the image. The image is still set as the background layers, which means it can't be moved, so we need to turn it into a normal layer. In the layer palette, right-click on the background image, and select Layer From Background.