The image of Christ "in another form" is a fresco by Manuel Panselinos (late 13th century) in the apse of the Prothesis of the Church of the Dormition of the Theotokos of Protaton "the mother of all athonite churches", in Karyes of Mount Athos.
The icon indeed bears an image quite different from other images of Christ in traditional Byzantine iconography. Despite signs of wear due to time, the image is very impressive. The head of the Lord is surrounded by a large halo-shaped Cross, His shoulders are broad, His hair lush and styled, and His eyes are piercing. Panselinos illumines the face with a bright red color to show a transforming in his form.
What is the source for such an image? Luke 24:13-33 relates the story of Christ's appearance to two men on the road to Emmaus. As the two men were talking about the events of Christ's crucifixion, we read: "As they talked and discussed these things with each other, Jesus himself came up and walked along with them; but they were kept from recognizing him." And towards the end of the conversation, as they broke bread and Jesus handed them a piece, it says: "Then their eyes were opened and they recognized him, and he disappeared from their sight."
This image therefore is the resurrected Christ as He appeared on the road to Emmaus, "in another form".