The login screen in Snow Leopard has a beautiful picture of an aurora. It’s colorful and awesomely technical. Lion changes that to a bland display called “linen.” If you want to change your linen, here’s how.

As of this writing, I haven’t found a utility that allows you to change the background of the Lion login screen, so you’ll have to do it manually. Here’s what you’ll need:

Administrator privilege.

A .png graphic the same screen size in pixels as your login display and at 72 dpi (or more properly ppi).

1. The first step is to determine your display’s resolution in pixels. To do that, go to System Preferences -> Displays and read off your currently selected resolution. For example, 1920 x 1200 or 1440 x 900.

2. Find a new background image you’d like to use. One source that I really like is MacDesktops. Download your favorite image that has the size from step #1.

The galaxy M81 from MacDesltops.com

3. Check to make sure the graphic has 72 pixels per inch (ppi) You can use a graphics tool like Thorsten Lemke’s Graphic Converter. You may need to scale the graphic back to the original size if the ppi needs to be changed. Also, convert to and save as .png if not already in that format. Graphic Converter can do all that — along with other popular apps. Here’s the info panel on the image I used on a 27-in iMac.

Info Panel from Graphic Converter

4. The location for the default linen image is:

/System/Library/Frameworks/AppKit.framework/Versions/C/Resources

You can navigate there in the Finder manually by drilling down or you can use the Finder’s Go -> Go to Folder… and paste this path in.

5. Locate and change the name of this default file from:

NSTexturedFullScreenBackgroundColor.png

to…

NSTexturedFullScreenBackgroundColor-orig.png

Enter your admin password when requested.

6. Rename your new .png graphic “NSTexturedFullScreenBackgroundColor.png” and then drag it into the location above. Again, you’ll need an admin password. You’ll have something that should look like this in icon view.

7. Log out, and you’ll now see your new graphic as the login background. If you see your image repeated and tiled, it’s probably because it’s either not the right size or wasn’t correctly changed to 72 ppi. My iMac’s login looks like this now.

I’m hoping that, very soon, some developer will build an app that automates this process. That way, we can change our dirty linen in an instant.