Documentation at https://www.novell.com/documentation/zenworks-2020/zen_fde_pba_override/data/b1b5fl2x.html describes some limitations of customization:
"The only way to replace the background image after it is in the Linux partition is to remove the ZENworks PBA and reinstall it"
"Do not copy the fsebrand.bin file from the older version and replace the fsebrand.bin file in the upgraded version of ZENworks. This file has functions that are specific to the current installed version, and replacing the file could cause issues after the upgrade."
I suggest 3 improvements:
1) support changing background without reinstalling PBA of any kind (preferred in next two steps)
2) unbundle background image from version specific parts in fsebrand.bin
3) add a bundle action to inject background image into PBA or provide documentation to do client-side on-the-fly injecting of pba_bkgd_image.png into fsebrand.bin. This would allow us to increase bundle version after Zenworks upgrade to reinject the customized backgound again