First of all, the idea that God cannot be seen and has never been seen is an New Testament Platonic innovation held by Paul and John/1 John. The OT doesn't say that. It says that no one has seen God and lived. But that's not true as the OT is full of verses contradicting this statement in Exodus, by stating that many have seen God face to face (and lived), etc.
But God also appears in disguises, as a burning bush, or as an angel (with Jacob), so the OT icons in question is not really heretical because the Father is depicted as an angel; it seems heretical because of the centrality and dominance of the Son, who, as Kolo says, supposedly deterrent to the Father in all things (which raises aproblem with the hypostatic co-equality)!
There are however, numerous, numerous truly heretical icons in supposedly orthodox churches, depicting the Father as an "old man".
Here is one example
The icon, second from the left of the Royal Doors, is a Trinity icon depicting the Father as an old man. This is in a Greek Orthodox Church and the icon was written in Mt. Athos, the Holy Mountain, in Greece, one of the holiest places in Orthodoxy!
Here is another Greek Orthodox iconostasis (icon stand), in Auckland, New Zealand, with the same icon (second from the left)
And I don't mean to pick on Greeks, as there are numerous "Trinity" icons in other Orthodox churches, especially Eastern-Catholic Ukrainian and Rutheaian variety), even Anglican!
It's amazing, but pictures don't lie. Whether you consult the Bible or the Church, you run into the same dilemma.