A4 Board for an A4 Picture, this gives rigidity and therefore unlikely to bend.
Now you need to protect the surface of the picture, do this by placing paper, tissue paper or transparent film larger than the picture and tuck it under and fix it to the underside of the board you’re using.
This stops the picture from moving around in transit.
It is also a good idea to place another sheet of stiff board or hardboard over the protected surface (again cut to the same size). Finally tape the whole thing together on all sides and corners.
You can also wrap it well with bubble wrap and mark it fragile do not bend on the outside.