In the ancient ceremonies involving animal sacrifice, blood was shed upon the ground and the feet of those involved in the rites became bloody. The blood of the sacrifice upon the feet became a symbol of the sins for which the sacrifice was offered.
The feet of those who walk upon the mountains crying peace are cleansed from that blood. Christ's washing of His Apostles' feet was to symbolize this cleansing which He alone could provide. He employs no servant to provide such a cleansing. (2 Ne. 9: 41.) These feet, washed by Him are, therefore, beautiful because they connote the sanctity of the one crying peace.
"Upon the mountains" because the mountain is nature's symbol of the ascent to God. The climb represents repentance and purification of the soul. When a person stands upon the top of the mountain, she appears to be part of heaven itself and no longer earthbound. Her profile is with the sky, symbolizing the completion of the ascent back to God.
It is beautiful. All of it is beautiful. All of it is a reflection of the purity and intelligence of God, whose ways are higher than man's ways as the heavens are higher than the earth. (Isa. 55: 8-9.)