Ubuntu – Grub Customizer doesn’t change image background

I use the latest version of Grub Customizer (3.0.2) and Ubuntu 12.04. On appearance settings I changed the background image and the font, but once I reboot the system they don't appear; there's always the default Ubuntu background image and the font seems a bit changed but not as expected.

I tried to resize and crop the image as described here: Why can't I see the image changed with GRUB Customizer? but it doesn't work in any case.

On /etc/default/grub:

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT="0"
#GRUB_HIDDEN_TIMEOUT="0"
GRUB_HIDDEN_TIMEOUT_QUIET="true"
GRUB_TIMEOUT="10"
GRUB_DISTRIBUTOR="`lsb_release -i -s 2> /dev/null || echo Debian`"
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL="console"

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE="640x480x15"

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID="true"

# Uncomment to disable generation of recovery mode menu entries
#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"

export GRUB_MENU_PICTURE="/home/attilio/Immagini/grub/1366.jpg"
export GRUB_COLOR_NORMAL="white/black"
export GRUB_COLOR_HIGHLIGHT="light-cyan/black"
GRUB_FONT="/boot/grub/unicode.pf2"

Thank you very much in advance.