correct tinypoodle, this is unreleated to syslinux because it happens with boot via grub, too.
This is solved. Here is the solution:
- despite wallpaper in /opt/backgrounds (in "mydata"), it did not come to the screen
- the wallpaper was activated in the wallpaper menu of the "control panel"
- then I made a backup with bootlocal.sh -b (just in case..) and now..
the wallpaper is coming automatically.