You would create a device backup with the appropriate wallpaper set, and then restore that to your iPads. Remember that you have to make separate backups for supervised and un-supervised devices.

If I'm understanding correctly, create a backup using apple configurator then apply casper profile? There's no way of doing this all wireless via casper mdm server?

No, not at this time. Even small changes such as lock screen text/custom image require the device to be Supervised in Configurator. Its a pain for sure.