Ubuntu – Can’t I see the image changed with GRUB Customizer

I installed grub-customizer to edit the boot-screen. It works so far (i. e. for timeout), but changes concerning the background image (and maybe font styles, too) aren't applied. I placed the image in /boot. How to improve that?

This is what /etc/default/grub says:

# 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="Ubuntu, mit Linux 3.0.0-16-generic"
#GRUB_HIDDEN_TIMEOUT="0"
GRUB_HIDDEN_TIMEOUT_QUIET="true"
GRUB_TIMEOUT="3"
GRUB_DISTRIBUTOR="`lsb_release -i -s 2> /dev/null || echo Debian`"
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nomodeset video=uvesafb:mode_option=1280x1024-24,mtrr=3,scroll=ywrap"
GRUB_CMDLINE_LINUX="splash vga=795 quiet"

# 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="1280x1024"

GRUB_GFXPAYLOAD_LINUX="1280x1024-24"

# 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"

GRUB_SAVEDEFAULT="false"
export GRUB_MENU_PICTURE="/usr/share/backgrounds/warty-final-ubuntu.png"
export GRUB_COLOR_NORMAL="dark-gray/black"
export GRUB_COLOR_HIGHLIGHT="white/black"