If you've never read the Television Without Pity website, I'm not sure I can explain it. But one of the things they do over there is a Pixel Challenge. It involves a topic, and minimal graphic design skills, and a warped sense of humor.
Check out the picture in the banner on the home page of the board (the one where they're all leaning against the wall.) Look at Gibbs face carefully, and you can see that half his mouth is turned up and the other half down.
Not only does this give him that endearingly rugged look that we all love, but if you cover up half his face at a time you can make him look alternately angry and happy.