For what it's worth, I don't see "JPEG artifacts" on my 20" mac, or my 15" mac laptop, so I honestly some of you are now just being critical to make yourselves feel better 
It's not about the size

Nowadays TFT displays have so big variety in quality and way they show colors and contrast etc.. I have crappy display which has a bit over brightness and those artifacts show up more easily than maybe on some better monitors with more natural contrast etc. In any case, the red color looks very varying here as well as edges in several places.
BTW, the header has NEVER been a .png
With that suggestion I wasn't referring to past, but was just suggesting that it could be better as PNG. And still think that PNG suits better for that kind of gfx.