This is one of the 13 mosaics of the life of Christ along the
upper band of the left wall of the nave. In Mark 2:1-5 and Luke
5:17-26 the friends of a paralytic try to take him to the house
where Jesus is staying, but it is so crowded that they cannot get
in. So they uncover part of the roof and let him down into the
house, where Jesus heals him and forgives his sins.
For the sake of compositional simplicity, Jesus and (perhaps)
John are placed outside on the left as the man is let down into
the house on the right. The curvy headboard suggests that what the
man is lying on is a bed (lectus, the word used in the
Vulgate Luke) rather than a pallet or camp cot (Mark's grabatus).
Also suggestive of Luke is the use of just two friends: Luke gives
no number; Mark says there were four. But again, simplicity of
composition may be what determined the artist's choice.
See also a view of the entire left wall with commentary on the iconography of Jesus and the apostles in these mosaics.
More of the Miracles of Jesus
This image in full resolution