In case anyone else is wondering: The French phrase can be translated literally as "a canvas requires a wall", or less closely, "its boundaries are important for every picture".
(I am not a native French speaker and just piecing this together with a dictionary.)
Not a single result in French also.
I know there's a (more popular?) saying that is very similar but can't remember it atm.
I asked around since my first comment and not a single person knew about it.
As you say, it's not immediately clear what is meant.