GIMP Plugin Registry

This script produces a two colors border. You can choose the border width in % of the smallest image dimension, and the inner border width as a % of the whole border (and the colors, of course).
This can seem a bit complicated, but if you have pictures of different sizes and want a same-looking border, % will get you there.

Just remove .txt from the filename after downloading the script (don't know why it was added..)

Why this is not already a menu item I do not know.

Does what it says, makes a selection to match your layer.