Ever since the public release of iOS 5 and the notification center which came with it, there has been an array of custom mods you can add to it. However the notification center background has quite a boring plain background. People may like this because all they want to see is their information but if you're like me, you like making everything pretty.
Custom NC Background is a simple and free Cydia tweak which enables you to have a custom background picture in the notification center. Once installed, you will find the settings for it under the "Settings" app. I recommend using images with a resolution of 640x960 but if your background is abstract or a pattern it shouldn't matter too much.
Wallpaper background - I know people are bound to ask, so here is the wallpaper I used - Purple Galaxy Wallpaper