The first thing you may notice once you’ve installed OS X 10.5 Leopard is that the default desktop picture is no longer the ‘Aqua Blue’ JPEG that is used for 10.4 Tiger. It is, instead, the star-field-and-Aurora-Borealis picture that is the distinctive branding for Leopard, and this desktop picture is the one you will see at the login screen. Since I do not use the Leopard picture as my wallpaper, I sought a way to change the default.
Open a Finder window and look into:
/System/Library/CoreServices/
There, you will find a file called ‘DefaultDesktop.jpg’. Replace this file with your desktop picture of choice; just remember to rename that to ‘DefaultDesktop.jpg’. Do a logout/login.
And, while we are on the subject, you may have noticed that black is not one of the colors you see listed in the Solid Colors section of the Desktop & Screen Saver preference pane; I noticed this when I had to hook my MacBook Pro up for a presentation and I was unable to set a solid black background for the desktop. The list of colors are Aqua Blue, Aqua Dark Blue, Aqua Graphite, Gray Light, Gray Medium, Gray Dark, Kelp, Lavender, Mint, and White. But no Black.
To remedy this, simply browse to:
/Library/Desktop Pictures/Solid Colors/
Open any of the PNGs in there in Photoshop, modify its lightness to black, and save it as a unique PNG.
34, male and very Cancerian. Often irascible, even if he is prescient to his emotions. Lives for the resonance of beautiful passages, cinematic magic and aural bliss. Dabbles in 
I found your tip via google. Glad to have found a person just as particular about the opening screen. After all, its the first thing we see every day!
Thanks. nice site, by the way
Thank you, Hui Min.
Since you’re particular about what you see on the startup screen, check out iQuip. I think you’ll like it.
Thanks for the tip, I found you via Google.
This is cool. Thanks man.