After the first bug zapper in the Lumbermill level, you have to pass a few saw blades. Then, look to your right to see a tunnel with a saw blade in it. Look to your left to see a stream. Jump off the platform to your left and hover, while staying close to the wall. After a few seconds, you will come to a small space in the wall. Inside is the picture. Smash it to find the Samos Mask.