Fo some reason, Terminal wasn't liking the "cd" before /Extra (no such file?). Found a simple solution on insanelymac that worked just perfect for me, though I'm glad to know about those terminal commands, so thanks again:
"just drag damn' file to your desktop. After that edit your file (the desktop one) and save it. To finish the job you drag the desktop version in the same folder where the old one was ... /Library/Preferences/SystemConfiguration/ and it should appear some dialog window....press the Authenticate button and type your password when it's asked for. Good job ... all done!"
So I dragged the boot.plist out of /Extra/theme/default onto the desktop (I was editing the plist from finder search or from the default folder), edited it, saved it, dragged it back into the /Extra/theme/default folder. Was asked to authenticate and did so. Presto! Dragged the apple boot png out and the png I wanted in, authenticated. Restarted and presto, a the apple logo gone above the spinning gear and the new png staring back at me. No why can't you edit the plist in its own location? Seems arbitrary that it allows itself to be edited on the desktop but not in its default location...