Background image manual grub.cfg

You need to add the next line to your grub.cfg before the menus. Since it is custom made and you're not going to use update-grub you shouldn't have any problem:

 background_image /boot/Your_image.png

Now, if you want to have a different background for your distros, you just need to add the line in the menu. For instance, this is the part where my menus start; as you can see I have a default background before the menus so that when grub starts it has a background, and after that each menu/submenu has its own background:

background_image /boot/SolusOS-splash.png

set color_normal=white/black
set color_highlight=black/white

submenu "Debian 8.1 -->"{
     submenu "Debian 8.1 i386 -->"{         
            background_image    /boot/DebianLava-splash.png         
            set iso=/boot/ISOs/debian-live-8.1.0-i386-gnome-desktop.iso
          ....

EDIT: Here are my search path entries (mind the uuid is my usb's ID) don't know if this might be your issue:

set boot_uuid=D042-8A53    
set root_uuid=D042-8A53

search --fs-uuid $root_uuid --set=root
search --fs-uuid $boot_uuid --set=grub_boot
if [ $boot_uuid == $root_uuid ]
then
    set grub_boot=($grub_boot)/boot
else
    set grub_boot=($grub_boot)
fi