The wire could be stainless steel, chain, nylon, kevlar or other material. If only one hook is use then the maximum force it has to bear is a downward force exerted by the weight of the picture. The real forces at play What is not apparent is the force that is created by the degree of tension in the wire. The tighter the wire the greater the force applied to the hanger through the wire. The breaking strain of the wire and the durability of the hanger and the way in which the wire is attached to the hanger must all be able to sustain the forces which are developed when the picture is hung on the wall.
Tight wire equates to more force being exerted on the hanging points and hooks.
This isn't just a feeling there are forces at play.