It's the Help file that's obsolete - the help file corrections usually lag behind the changes to the commands.
The texture backdrop command was reinstated by popular request in a "recent" upgrade. It's always worked for me.
Edit
The following code works fine for me - just press the spacekey to clear the backdrop.
set display mode 640, 480, 32, 1, 8, 0
sync on : sync rate 60 : sync
autocam off
position camera 0, 0, -300
point camera 0, 0, 0
load image "tiles3x2.png", 1 ` a 2048x2048 image
texture backdrop 1
make object cube 1, 100
repeat
rotate object 1, object angle x(1)+0.2, object angle y(1)+0.3, object angle z(1)+0.5
sync
until spacekey()
while spacekey()
endwhile
texture backdrop 0
repeat
rotate object 1, object angle x(1)+0.2, object angle y(1)+0.3, object angle z(1)+0.5
sync
until spacekey()
end
Just replace the image with your own image file. This code is a trivial extension of code posted on the same topic a few weeks ago.