I have not looked at that guide, I do not have to as changing thatsetting has never been a good idea.
The main reaso nit is not a good idea, as you have discovered. is that it does not work.
Now, have you installed "grub2-splashimages". This is not really needed but it does give you another folder in /usr/share/images.
Your 05_debian-themes file calls for /usr/share/images/desktop-base. That needs to be changed if you want to use the files from /usr/share/images/grub that is installed and populated when grub2-splashimages is installed.
If you look at the link in my sig the top 2 links in my post are the best. I use the directions from the first on (mostly).
I have found, though, that just using the desktop-base file is fine if you want to use your own or some found file. You do need to change (maybe) the font colors in the lines below where you put your image.
Make sure that the / is before your image name and the . after it. Png files work fine.
# check for usable backgrounds
use_bg=false
if [ "$GRUB_TERMINAL" = "gfxterm" ] ; then
for i in {/boot/grub,/usr/share/images/desktop-base}/widget.{png,tga} ; do
if is_path_readable_by_grub $i ; then
bg=$i
case ${bg} in
*.png) reader=png ;;
*.tga) reader=tga ;;
*.jpg|*.jpeg) reader=jpeg ;;
esac
if test -e /boot/grub/${reader}.mod ; then
echo "Found Debian background: `basename ${bg}`" >&2
use_bg=true
break
fi
fi
done
fi
# set the background if possible
if ${use_bg} ; then
prepare_grub_to_access_device `${grub_probe} --target=device ${bg}`
cat << EOF
insmod ${reader}
if background_image `make_system_path_relative_to_its_root ${bg}` ; then
set color_normal=black/black
set color_highlight=white/black