According to libreboot.org, png's will do the trick: Changing the background image in GRUBUse cbfstool from libreboot_util, or libreboot_src/coreboot/util/cbfstool/ if you want to build from source.
$ ./cbfstool yourrom.rom remove background.png -n background.png $ ./cbfstool yourrom.rom add -f background.png -n background.png -t rawWhen you’ve done this, re-flash your ROM and you should have a new background at boot time.
https://libreboot.org/docs/grub/index.html