There's a sign that Christ does with his hand in many pictures- what does it mean?

Those pictures are paintings, not photographs. During the Middle Ages, artists were commissioned to paint or sculpt persons in the Bible. The Church requested that pictures and statues show gestures or postures that reflected different Church concepts and traditions. Some traditions were taken from ancient Roman and Greek philosophies. They were used to 'teach' the illiterate churchgoers traditional Church teachings. These gestures have nothing to do with God's Word or Jesus' teachings.