The verse in Isaiah 30:20 says, "And your eyes will see your teacher." This literally means that in the time of the final Redemption, we will literally see God.
However, throughout the generations our sages have also explained this verse in practical terms: to keep the image of the teacher in mind while studying and at all times. Some sources:
Shekalim 7b, Rav Guidl said: He who speaks a teaching on behalf of another, should see as if the other were standing in front of him.
The Korban HaEda commentary explains: the main learning is by observing his face as it is written "And your eyes will see your teacher.".
Eiruvin 13b: Rabbi says that because he saw Rabbi Meir's back he is so wise… Rabbeinu Chananel explains that Rabbi was not sitting in the front rows of Rabbi Meir's classes; if he had been "facing him," he would have been wiser, as it is written, "And your eyes will see…""
The Chida writes (Midbar Kdemot entry "Tziur"): When a person imagines a holy image, that image completes their intellect. Therefore, the Zohar (Mishpatim 123b) says that Rabbi Aba always had the image of Rabbi Shimon present… and thus grasped great matters. Similarly, the Arizal wrote that when a matter in Torah is complex for him, he imagines the image of his teacher, and it helps him understand the matter.
The Chida (Torat Chida, Terumah 33, in the name of the Arizal) also writes: Whoever wishes to succeed in his studies and understand should imagine the face of his teacher, and thus the portals of light will open for him. Therefore, he should imagine the face of his teacher when studying his Torah, and improve his conduct in thought, word, and deed, because his teacher observes him and his activities, and in the future he will have to give an account to his teacher for all his actions.
For this reason, many good people possess pictures of Tzadikim (righteous ones) and place them in their study homes, etc., to fulfill the verse: "And your eyes will see your teacher," just as the Chofetz Chaim had a picture of the Tzadik Nechumke of Horodna, and looked at it every day.
The Rebbe said of the previous Rebbe: Those who were in private audience with the Rebbe, imagine the Rebbe's face as if you were now entering an audience. And those who did not see the Rebbe, imagine him through a photograph.