https://www.tomshardware.com/how-to/custom-raspberry-pi-splash-screen
1. In your favorite image editor create a splash screen and save it as splash.png. We chose a 1920 x 1080 resolution as it scales well with 16:9 resolutions.
2. Copy the image to a USB flash drive and insert the drive into the Raspberry Pi.
3. Copy splash.png from the USB flash drive to your home directory /home/pi/
4. Open a Terminal and change directory to the location of the original splash screen for your Pi.
$ cd /usr/share/plymouth/themes/pix/
5. Backup the original splash.png file to a file called splash.png.bk.
$ sudo mv splash.png splash.png.bk
6. Copy your splash.png to the current directory. This will copy the freshly created splash image ready for use.
$ sudo cp /home/pi/splash.png ./
7. Reboot your Raspberry Pi.