I’m on Debian with XFCE I’ve already tried modifying /etc/default/grub and /etc/lightdm/lightdm-gtk-greeter.conf The first resulted in nothing (I actually have grub timeout set to 0, so I’m not sure if I only need changing grub’s config), the bootsplash was still default The second resulted in black login bg
I think the boot splash is done by a software called Plymouth. At least that used to be the case when I last had a boot splash: https://wiki.debian.org/plymouth
As for the login greeter… That could have a bunch of different reasons. Maybe you have a typo in the config. Maybe the picture has the wrong format. Or the file system permissions are wrong hence it can’t be loaded?!