Technically, if one goes by the ruling at the Second Council of Nicea, depictions of the Father are a no-no.
Interestingly, one almost never sees iconic representation of the Father in Eastern iconography. But the West has been more...loose about this.
A triangular halo is given the Father in art, as others noted, to denote the Trinity.
Because it's not appropriate to depict the Father directly, the Father is also often symbolized in iconography and art in other ways, usually as the "Hand of God".
Another use of the triangle is when it surrounds the Eye of God, the triangle again symbolizes the Trinity and the eye symbolizes God's providence and sovereignty.
-CryptoLutheran