For those that didn't have a profile picture taken right at the beginning (and therefore couldn't use the above methods without reloading the ChromeOS), good news!

With the current version, users are offered the option of changing the picture in the Chrome settings. As long as the desired picture is in the File Shelf (Downloads directory), it can be used and it automatically creates the PNG image with the filename in proper syntax.

Chrome OS 0.12.362.2 (Official Build) dev-channel x86-mario
Google Chrome 12.0.723.1