background image on a usb-stick-grub (without update-grub)

[c.buhtz]

I have a Grub2 installed on a USB-Stick.

This is the grub.cfg (created by myself!)

insmod PNG
set gfxmode
background_image /boot/grub/background.png

menuentry "Siduction LXQt" {
    insmod iso9660
    insmod part_msdos
    insmod ext2
    search --no-floppy --fs-uuid --set=root DF5C-D732
   
loopback
loop /iso/siduction-15.1.0-paintitblack-lxqt-i386-201601171445.iso
linux (loop)/boot/vmlinuz0.686 fromhd=UUID=DF5C-D732
fromiso=/iso/siduction-15.1.0-paintitblack-lxqt-i386-201601171445.iso
boot=fll lang=de_DE tz=Europe/Berlin noplymouth noeject md5sum toram
initrd (loop)/boot/initrd0.686 }

I don't have update-grub here. I don't want to and I even don't know
how to make my update-grub touch the usb-stick on sdb. Update-grub only
toches the current booted hda (Ubuntu 14.04.3).

Two problems here:

1.
The resolution is not set correctly. It is only the low standard
terminal resolution.

2.
And (I think) because of that the background image is not set.

Any idea?