You can use the images that come with Zeppelin or you can just make your own...
Image Dimensions : Width-140 pixels Height - 28 pixels
Make a folder, Choose an image(.png) (with the correct dimensions) and copy 10 into the folder renaming each one :
(for Retina type devices you only need 5 images renaming each marked in green)
Using any software tool you like(WinSCP,Cyberduck,iFunbox....) SSH into your iDevice and copy the folder containing your carrier images to Library > Zeppelin.....then exit.
On iDevice go to Settings>Zeppelin>Theme...select your theme, back to Zeppelin and press Apply & Respring
Here's a quick 5 minute image I made...