No ground wire connected to the 3-way switch
You could either use a self grounding switch, or use the other grounding screw to attach a pigtail out to the new switch.



To create the pigtail:


Get a short length of green or bare wire, the same size as the largest wire in the box.
Attach one end of the wire to the box using the screw highlighted above (remember to wrap the wire clockwise around the screw, and tighten the screw).
Attach the other end of the wire to the grounding terminal on the new switch (again wrapping in a clockwise direction, and tightening the screw).