According to libreboot.org, png's will do the trick:

Changing the background image in GRUB
Use 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 raw
When 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

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