I may add images to this tut if you want them!
1: Create a
new layer above your sig image name it border
2:
Select all and fill with white
3:
Shrink the selection by 2 and fill with black
4:
Shrink the select by 1
5:
Clear the selected area
6:
Deselect 7: From here you can play around with the layer mode until it looks how you want it!
Enjoy and happy gimping