Comment 2 for bug 1317236

FYI for anyone else who comes across this bug report, I was able to get around the issue in regular Ubuntu 14.04 by overwriting /usr/share/icons/gnome/256x256/status/avatar-default.png with my desired .png (the greeter even resized it properly). You have to run 'gtk-update-icon-cache /usr/share/icons/gnome' as root after the change and make sure the new /usr/share/icons/gnome/icon-theme.cache file has 644 permissions.

Do this at your own risk. I don't know where else that avatar-default.png file is used.