After backing up your system with systemback, you're left with a .sblive file, open this archive in xarchiver or similar
go into syslinux folder and extract 'splash.png' and 'syslinux.cfg'
(systemback will rename this to isolinux/isolinux.cfg during ISO conversion)
replace splash.png with your image (640x480 png)
edit syslinux.cfg to match your fork/respin name (change the lines starting with "menu label" and "menu title")
add new files to archive
(if you don't wanna change the label, just replace the splash.png in /usr/share/systemback)
open systemback, select "Create Live System" then "Convert To ISO"
Screenshot: i.imgur.com/Pjl6FPI.png
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