The default pattern for Lion’s Dashboard graphic is a rather bland Lego pattern. There’s no system setting to change it, but you can do it yourself. All you need is Administrator privileges.
The default Dashboard, Lego pattern
In a previous tip, I described how to change Lion’s login screen. This technique is similar — you’ll need admin privileges to replace a system file.
1. Select an image that you want to use for the Dashboard background. It needs to be the same size as your main display. Alternatively, it can be a symmetric image that can be tiled in a regular pattern. Apple’s lego file is 20 x 20 pixels.
2. Unlike the login screen, however, this image doesn’t have to be 72 ppi. It can be, for example, 300 ppi. However, it must also be in .png format. Again. I recommend the images at MacDesktops.com. You can use an app like Graphic Converter to alter and verify that the image has the same resolution as your display and convert to .png if necessary. Finally, name it “pirelli.png”.
3. In the Finder, navigate to this location:
Look for the application Dock.app. Right click it and select “Show Package Contents.” Open the Contents folder and navigate to and open the Resources folder. Scroll until you find the file “pirelli.png”. Rename it to something like “pirelli-orig.png”. You’ll need an admin password to do that.
4. Now drag your new image into that same Resources folder. Again, you’ll need enter an admin password. It should look something like this, in icon view.
5. Next, you need to reboot the Dock like this, in the terminal.
Note that “Dock” is case sensitive. You’ll see the Dock disappear, then reappear. This is really easy, but if the Terminal.app intimidates you, you can achieve the same thing by logging out, then back in.
If all went well, if your small image scaled to your satisfaction or your large image was exactly the screen resolution (or larger) and in .png format, you’ll see the result the next time you launch Mission Control and select the Dashboard.
New Dashboard background
As always, if you find any tricks or exceptions, discuss in the comments below.
Addendum: this technique worked fine on a MacBook, 1440 x 900. But I had cosmetic problems with an iMac at 2560 x 1440.