I’ll assume you’re running PureOS, or another distro with flashrom 1.0+ installed, and have an open terminal to run these commands:
make a tmp dir and change to it:
mkdir ~/bootsplash && cd ~/bootsplash
download a precompiled cbfstool binary (can also compile from coreboot source):
curl -LO https://mrchromebox.tech/files/util/cbfstool.tar.gz
extract it
tar -zxf cbfstool.tar.gz
read the firmware from your device and get checksum
sudo flashrom -p internal:laptop=force_I_want_a_brick,ich_spi_mode=hwseq -r coreboot.rom
sha256sum coreboot.rom
do this 2x more and ensure that same checksum generated all 3 times before continuing
remove existing bootsplash:
cbfstool coreboot.rom remove -r COREBOOT -n bootsplash.jpg
(optional) add in a custom bootsplash (file format/resolution restrictions apply here, I don’t have them handy unfortunately. Use same as existing to be safe?)
cbfstool coreboot.rom add -r COREBOOT -n newbootsplash.jpg -t raw
write firmware back to device:
sudo flashrom -p internal:laptop=force_I_want_a_brick,ich_spi_mode=hwseq -w coreboot.rom
reboot when finished