If you don’t know how to SSH into your iDevice, visit this post to learn how.
1. Open up a photo of your choice in a photo editor of your choice.
2. Resize the photo width to 640 and the length to 960 (640×960).
3. Save it as a .png and change the name to BulletinListLinen@2x.png
4. Now open up and login to WinSCP or CyberDuck.
6. (Optional) Drag original wallpaper to the desktop and copy it, just in case.
7. Locate the photo you saved earlier in step 3, drag and drop it into the folder. Click “copy”, then click “yes”.
8. Respring and your are finished, once you followed the steps correctly. Your notification Center wallpaper should be the one that you chose.
Credit to the guys over at iDeviceDaily for the info.