I think the blue line represents the part of the defenders body closest to the goal and the red line indicates the part of the attackers body closest to the goal. But for the attacker, as the other poster pointed out, it has to be a part of the body that you can score from for it to be offside. Therefore, the image above shows that Casey was not offside as he cannot score from his elbow, and his shoulder, which he can score from, is in line with the blue line (defenders toe or knee), which makes him onside. Another image from this decision shows that the lines were drawn once the ball had left the boot of our man who took the free kick. If so, this offside and no-goal is another incorrect decision that has gone against us.