Not to burst your bubble or anything, but we know. There are plenty of instructions for how to do this, but I personally don't like digging around in the System directory or messing with the Terminal, so I downloaded a wonderful tool called Visage. Visage "allows you to easily customize your Boot Panel, Boot Strings, Login Panel, Login Screen Background, Dock Poof, and Dock Transparency." I love it; my boot panel is a frog, my login background is the sky w/ seagulls flying, and my login panel is the one Visage comes with that has the apple logo as the nucleus of an atom. I had TransparentDock before Visage, so I don't use Visage for that, but being able to easily change my Dock Poof is really cool. You can use any poofs from ResExcellence, which is nice.