Mac: Notification Centre is one of the best parts of Mountain Lion, but the classic linen background is a bit bland. If you want to add your own image, Cult of Mac shows us an easy way to spice it up.
To use a custom image, you'll need to pop into the Notification Centre app. To do this, head to System/Library/CoreServices, right-click the Notification Centre app, and select "Show Package Contents". Inside the Resources folder is a file called linen.tiff. This is your Notification Centre background. Save the linen.tiff file to another location (so you can restore it if you like). Then 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 Centre. When you open up the Notification Centre panel again, your new image will be there.
Change OS X's Notification Center Background [Cult of Mac]