Fist, my comment about the corner icon was, of course, mostly in jest - even if it does, in a very small part, illustrate that size optimality isn't something that everyone pays a lot attention to. (Not that it's a huge deal, really, just a fact of life.)
Second, amusingly the header.png you link also has a useless alpha channel, and optipng -o7 can shave off a whole 17131 bytes (16.22%) off of it. There are probably a lot of such pictures out there - I think gimp produces pngs with empty alpha channels by default when the source image has it, even if the exported (possibly flattened) bitmap doesn't require it. I wonder if that should be a bug.
Anyway, optipng is a nice tool to run at least for images that'll be served lots and lots of times. Even if you don't use the slow, -o7 "obsessive-compulsive" mode, it can often improve on the compression settings a bit. More importantly, it will remove that spurious alpha channel and also do other color type/bit depth/palette reductions where appropriate.
(Being myself a tad OCD, checked that the header.png "only" reduced by 16976 bytes or 16.07% using optipng's fast 8-trial default settings. For comparison, -o7 runs through 240 trials of different setting combinations.)