After a boot entry has been selected in GRUB2, for a short time (1-2 sec) another screen is displayed which shows:
Loading initial ramdisk ...
I found out how to suppress this output (by setting the variable quiet_boot in /etc/grub.d/proxifiedScripts/linux to "1"), but the splash screen with Debian logo still appears.
Is there a way to completely prevent this screen from being shown, or at least replace the background image with a custom one?
I found the answer finally! The mentioned screen/image which I want to remove is displayed by the so called desktop-base package. On this german Ubuntu Wiki page it is recommended to uninstall this package if someone wants to customize the graphical GRUB menu on his own, because otherwise images from this package will still be displayed (which was the case on my system). The following command prevents the screen from being displayed any more:
dpkg -r desktop-base