[syslinux] Fn textfile background

[Ady]

Hello,

In doc/menu.txt:
"F1 textfile [background]"

I am trying the second optional parameter using 640x480 x 4bits color 
depth png's.

Should I be using a different file format?
Or a different color depth?
Or specific colors?

Which versions are supposed to support the second optional parameter, 
"background", from menu.c32?

I have tried (and failed) with (menu.c32) v.3.86 and v.4.07.

Or, perhaps this documentation is incomplete / wrong?

TIA,
Ady.