Framed artwork combines a fragile glass front with a frame and a delicate print or canvas behind it, and each needs protection. Broken glass can slice the art it was meant to protect, so securing the glass is priority one. Packed upright in a proper picture box, framed pieces travel safely no matter the size.
Pro tip: Pack several flat pieces vertically in one picture box, separated by cardboard, like records in a crate.
Watch out: Broken glass can slice the artwork behind it. Always tape the glass so shards stay put.