you can extract filesystem.squashfs from the iso or depending on how you made your live usb at usb or iso\live\filesystem.squashfs by using unsquashfs patch to file\filesystem.squashfs and search through the files till you find where the backgrounds are and modify them than use squashfs to rebuild filesystem.squashfs or you could test and see if your settings stick on your persistant partition. hopefually i was some help
Smile while you can for in the future there my be nothing to smile about.
申し訳ありませんが、これは翻訳することができませんでした。