I agree on those complexity graphs since they're actually scanned from Sean Carroll's book and edited in Pixelmator - if I was better with graphics I could probably recreate them as SVG or in an editor and make it only a couple kb.
Same for the MinutePhysics screen grab, though I couldn't bring myself to attempt a poor recreation of their work and couldn't get rid of the weird pink gradient background which probably prevents better compression.
The Youtube cover images are still being pulled in from youtube - I debated downloading/compressing/hosting them myself, but figured I'd gain more from downloading them from the separate domain since I still think there is a default limit of connections the browser will open to a single domain at the same time.