I presume you are talking about the plymouth theme background.
The simplest way to change the background of the plymouth theme is to replace
the /lib/plymouth/themes/trisquel-logo/background.png with you're preferred ;
you'll need to be root to do that; and to take effect you'll need to run
"update-initramfs -u" (sudo update-initramfs -u).
For the GDM3 background you can do the same with
/usr/share/backgrounds/toutatis.jpg, to take effect you'll just need to
reboot or reload GDM3.
Hope this helps.
