Because he is the image of the invisible God (Colossians 1:15).
Consequently, his image is the image of the image of the invisible God. God forbade graven images in the Old Testament because God had no image then, being pure spirit but when God took on human flesh and became man, he took on an image as well, launching a whole economy of images. Now God has an image, his Son, Jesus Christ. Jesus predicted this when he said, "Just as Moses lifted up the snake in the wilderness, so the Son of Man must be lifted up" (John 3:14). He was comparing himself to the bronze serpents God commanded the Israelites to make so that they could be cured from the bites of serpents. Anyone who gazed on the bronze serpent would be healed and live. So anyone who gazes on the crucified (lifted up on the Cross) Jesus Christ will be healed from the Ancient Serpent's wound (cf. Genesis 3:1-6, Revelation 20:2).
This is why we have crucifixes.
Galatians 3:1 indicates that even the early Christians gazed upon portrayals of the Crucifixion such as a crucifix.
Here are some previous postings with answers we gave on the subject:
Hope this helps,