How to get a higher res image in the Login page?
Unfortunately, you're right. The icons are just low-res, not up to snuff for how they are being rendered now. You can find them, and others, in this folder: /Library/User Pictures/Animals. Funny thing is, some of the icons, like the Zebra or Penguin, are much higher res. It seems Apple just added new, better icons without updating all of the old ones. However, if you were to replace the cat icon with another, higher-res icon of the exact same name, I'm sure it would replace it just fine.