Coreboot

Coreboot is a Free BIOS replacement. You may have heard the myth that unlike libreboot Coreboot contains non-free blobs but this is not entirely correct. The trannies have lied to you. Coreboot comes without blobs by default on machines that don't need blobs and only comes with the blobs required on machines requiring them. This allows it to support a large number of motherboards. Libreboot was created because the fsf hates everything that even gives you the option (the freedom) to install proprietary software. Coreboot is just as free as Libreboot on machines that support both.

Payloads

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[1]

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.