Here, Plymouth theme uses a theme framework to display the background or animation or the both, so its not possible to add a wallpaper(like in changing ubuntu login screen) to your boot screen, but the ubuntu repository comes it self with several good boot screens(Plymouth themes) which you can install.
- Ubuntu OS
- 5 minutes of you valuable time.
Step #1: Open up your terminal and enter the following command.
This will install Plymouth themes.sudo apt-get install plymouth-theme-*
Step #2: Now use this command to get a list of themes to select.
sudo update-alternatives --config default.plymouth