Okay, figured this one out myself, too.
For the Odroid Go Advance the custom splash screen .png graphic doesn't go into '/boot/' but in '/' (= root directory of the 'BATOCERA' partition on your SD card)
However, the image image needs to be rotated 90 degrees (counter clockwise) to be displayed correctly. (Apparently same issue as with https://forum.batocera.org/public/d/3284-c64-emulation-issues-rotated-screen-etc and https://forum.batocera.org/public/d/3289-retroarch-menu-orientation)