One way is to make a new layer, then select -> select all, fill with color, select -> modify -> contract by one pixel, then delete. Simple, easy border.
a good, cool and easy border to do is called the 3 Pixel Border simply make a new layer above the rest of your layers, and push Ctrl-A.. this will select all around your signature now you go Edit -> Stroke choose Black, 3 Px, and make sure Inside is checked... click OK repeat again Edit -> Stroke this time choose the color of your signature, and make it 2 Px, again with Inside checked... click OK repeat again Edit -> Stroke choose Black, 1 Px, Inside checked.. click OK should have a Black, Color of your Sig, Black border too it.. very effective for an example look at my top sig