There is a qualitative difference between Deity worship and Idol worship as Śrīla Prabhupāda explains below…
Woman guest (2): Could you… Do you think you could explain to me about the Deities and how it’s different from idol worship? Because no one has been able to explain that to my understanding.http://prabhupadavision.com/
Prabhupāda: Idol means your imagination. And Deity is not imaginary. Deity is installed by the authorized person and it is worshiped according to authorized methods. So it is not idol. Idol worship, you imagine something and, some doll or idol, and do in your own way, that is idol worship.
Puṣṭa Kṛṣṇa: She’s saying but it’s manufactured, it’s made, graven.
Prabhupāda: That doesn’t matter. But it is made according to the Vedic principles. Just like in the Vedas, Kṛṣṇa’s form is described, veṇuṁ kvaṇantam aravinda-dalāyatākṣaṁ [Bs. 5.30]. Kṛṣṇa is playing on His flute, veṇuṁ kvaṇantam. His eyes are like the petals of the lotus flower.
So if you follow the description, then it is coming from the Vedic literature or Vedic knowledge. It is not that an artistic way of imagining some idol, the eyes are like this and He must play flute. It is not like that.
Woman guest (2): That’s because it came from scriptures instead of from men’s minds.
Prabhupāda: Yes. Therefore it is authorized. When you take anything from the statute, book, that is law. And if you manufacture something, that is not law. Just like, I’ll give you one very good example. Just like in your front of your house there is U.S. letterbox.
So another man, he sees that the box is in front of his house, “Why shall I go so far? Let me manufacture a box here.” So he’s posting. After six months, he’ll see all these letters are lying there. (laughter) And one who is posting that authorized box, his letter is going to thousands and thousands of miles away. So you cannot imitate. If you imitate U.S. letterbox in front of your house and post your letters, it will never go, it will remain there.
[Evening Darśana: July 8 1976]