I'm not sure it works as a whole. IMO, the border is way too thick and the image itself seems to end very abruptly. On a side note, I have that comic; 'tis great.
*Border size decreased from 8px to 5px* Any better? The reason behind the thickness of the border is because the background is white. I personally think that the black border really plays a good role in the sig since the background is white.
That's a much better thickness. Maybe if you added the original caption into the sig. There's a lot of blank space.