Are you allowed to use pictures in your Engineering Notebook? If so, those with experience, do judges prefer pictures or drawings? Does it matter which you use? Is a good mix of both things better?
Yes, allowed and encouraged.
You should use a combination of both. Drawings are good for both initial design concepts and thorough schematics of completed mechanisms, but pictures (ideally in color) are the best way to really showcase your robot.
They do like to see pictures, and pictures will probably improve your score, but drawings are also very important. A mixture of both would be best.
Don’t forget to describe in WRITING how the drawings/pictures relate in terms of your design process.