It could get re-used as part of the logo on the main y-combinator site, and the same logo could be cached ;)

But then again, HN itself is already easily fast enough, and you don't optimize when you don't have to.

But probably not worth the effort - the extra CSS might mess up the equation.

its a very HN type of thing to do...code an image, that is

Looks pretty. Pretty things are nice. Nice things are good.