Re: [Trisquel-users] how do I change the GRUB background image?

[jbahn]

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