We are still actively working on the spam issue.
Coreboot by default use the simple as fuck SeaBIOS payload. Which is probably the closest thing to a stock BIOS. This payload supports a custom splash image. You can get some here They have to be saved in a weird way in GIMP
The other payload that can be used is GRUB, this is used to have Full Disk Encryption. This is the payload libreboot use for their most releases.