Gtfo GIMP users. This is NOT for you (mainly cause I'm not bothered to figure out how to do it on GIMP).
1. Open Photoshop!
2. Stop reading this tutorial and take a minute to regain some lost common sense from the Dead Sea.
3. Open your sig file, it doesn't really matter if it's in the PSD format or not.
4. Make 3 new layers above all the other layers.
5. Go to the lowest positioned layer of those three, make a selection of the entire canvas using whatever Marquee tool you want.
6. Go to Edit/Stroke.
7. Width: 1px Color: Black Location: Inside (That sounds gross to all you sick minded people), and click OK. Mode:Normal Opacity:100% and the unchecked Preserve Transparency box should be default.
8. Now go to the layer above which should be the middle layer (aka 2nd) of the three you made. Take another minute and inhale through the nostril just observing how common sense really is the human's 6th sense.
9. Repeat step 6, NOT 7.
10. W: 2px C: White Location: Inside OK
11. Now go to the final layer of the three (the one on the top).
12. Repeat step 6, I don't mind if you repeat step 7 because it's fun watching people comment saying they don't know what they stuffed up when they were only a few steps from finishing.
13. W: 3px C: Black L: Inside OK
14. Set the upper two layers (the one you just edited and the 'white' one, no that's not racist) on overlay.
I finished on 14 yet again, yays!.