There are many differences between the Orthodox and Catholic Churches but usually it is a difference of emphasis rather than (in my view) serious substance. Most often the differences are complementary. We need both views to understand more completely.
The most common subject dominating the apse wall behind or over an Orthodox altar is the Holy Virgin of the Sign or a slight variation of it –the Madonna holding the Christ Child in her lap. In Western (Catholic) churches the single most used subject is the crucifixion. Protestant churches almost always employ the plain cross.
The different imagery reveals the different emphasis each places on the same economy of redemption and salvation.
In the East it is the Incarnation as a whole that effects redemption. Man was redeemed when God took on human flesh, lived a human life in all respects except sin, suffered the worst death, and resurrected and ascended back to the Father. With the Incarnation, human flesh became capable of deification (sanctification). It became possible for man to enjoy eternal life with God in heaven. The icon of the Incarnation in the East is the Virgin of the Sign or the Holy Virgin holding her Son in her lap. It is the Eastern iconic image of redemption.
The Western Church does not believe something different but she most often chooses a specific moment, the moment of Christ’s horrible death, to symbolize the redemption. Catholics and Protestants emphasize an atonement theology as part of the Incarnation; that Christ paid for our sins through His sacrificial death. This is especially true in Protestant churches.
Catholic churches, in addition to the crucifixion, have a very rich tradition of highlighting other images of redemption (including images of the “Virgin of the Sign”) behind or above the altar, such as scenes from the life of Christ and images of the sanctified –the saints.
The Eastern Church chooses to display as its most important icon in the church building an image that views redemption as a whole piece. In the Western tradition redemption appears as a crescendo climaxing in the sacrifice of the cross. Neither emphasis diminishes the importance of the other. We should see them as complementing each other –”breathing with both lungs”.
We Westerners, especially Protestants, may be puzzled or even scandalized by the prominent emphasis of the Holy Virgin in Eastern Orthodox or Byzantine Catholic churches until we realize the meaning behind the icon.