Two ways, both involving getting around OS X's objections But most of the Aqua graphics are just pdf files generated by Photoshop, so they're easy to open if you know where they are and bypass the normal OS X.
Way one: Easier, less intimidating:
Start up from OS 9, so OS X won't bother you about this. Launch Photoshop, Cmd-O, and navigate to: System > Library > Core Services > Dock.app>Contents>Resources folder on the OS X disk. Poke around there; there are a number of files that Photoshop can open.
Important caveat: Work on a copy of the file, always! If you want to hack your system, you can save the new file under the same name. If you do, you
will hack the system. Don't wanna do that? Simply save the copy under a different name, in a different location (Like your Home folder)
Way two: Scarier, geeky, Unix-oriented:
Log into OS X as the root user, then follow the above steps. No, I'm not going to tell you how to do that
If you already know how, and understand the risks, be my guest.