Notification Center is one of the best parts of OS X Mountain Lion, but the classic linen background is a bit bland. If you want to add your own image, Cult of Mac shows off a pretty easy way to spice up your Notification Center.
To use a custom image, you’ll need to pop into the Notification Center app. To do this, head to System/Library/CoreServices, right-click the Notification Center app, and select “Show Package Contents.” Inside the Resources folder is a file called linen.tiff, this is your Notification Center background. Save the linen.tiff to another location (so you can restore it if you like). Now, you just need to pick out a new image, rename it to linen.tiff, and drag it into the Resources folder. When you’re done, open up Activity Monitor (Applications > Utilities), and quit Notification Center. When you open up the Notification Center panel again, your new image will be there.
Change OS X’s Notification Center Background | Cult of Mac