This program only affects Grub and the boot sequence. (Well, you could argue that it can add kernel parameters that will influence how the OS runs.)
The graphic images are only for the initial boot screen - what you see behind the menu entries on the Grub screen. Grub runs before any OS is loaded; plymouth is loaded and comes into play only after Grub passes control over to the kernel/OS. Splash images you see later on in the boot process aren't controlled by Grub.