Deities and Deity worship
All scriptures are from Krishna, but not all scriptures are equal, just as there are math books for both second grade math students and there are textbooks for those studying calculus. All are correct, but one is for those with higher understanding.
Deity seva requires deep understanding. Worship is not casual or arbitrary, nor is it meant to present the Lord with our wish list, but rather it is dictated in the scripture and intended for the service and pleasure of God Himself. According to the various positions of people in the modes of material nature and according to their spiritual advancement various opportunities are given so that all persons can eventually be elevated.
In the 17th chapter of the Bhagavad-gita it is explained in detail how different persons worship differently due to the modes of nature they are affected by. But the results of their worship are not the same. Those who worship demigods go to the planets of the demigods, those who worship ancestors go with them and those who worship Krishna, God, go to the spiritual world.
In Islam and Christianity the goal is heaven, but heaven is not necessarily the highest goal. Going to Goloka Vrindavan, having a personal and loving relationship with the Lord is higher than having material opulence, beautiful women serving you, and enjoying heavenly sense gratification. Those are not the goals of one practicing Krishna consciousness. The goal is to become an unalloyed lover of God and serve Him for His pleasure.
Worship of "graven images" would include worship of any unauthorized form of the Lord—or something which is really not the Lord—as the Lord. We do neither of these things. Krishna has many forms, which He Himself speaks about in the Srimad-Bhagavatam, which is considered the highest and most complete of all scripture.
It is totally sufficient to worship Krishna alone. Lord Krishna incarnates in different forms... Chaitanya Mahaprabhu, Sita-Rama, Krishna Balaram, Jagannath, Nrsimhadeva are all Krishna Himself, they are simply different forms that He appears in for different reasons.
In the Srimad-Bhagavatam, verse 1.3.28, it's explained that all these incarnations have Their source in Krishna. He's like the original candle that is used to light all other candles. They're all candles, but still there's one original. We don't worship demigods or "others"—we simply worship Krishna in various incarnations.
We offer respect to Srila Prabhupada, our guru, because we are indebted to him for giving us knowledge of Krishna, but we don't worship him as God Himself.
Worship of Krishna's devotee is done according to Krishna's direction, and that direction is given and understood through a guru or teacher. The same principle applies in all traditions; Muhammad was a teacher of Islam, the Rabbi is the teacher for a Jewish person, the priest or even Christ himself is the teacher for the Christian, and the Pope is the teacher for the Catholic. Trying to interpret scripture based on one's speculation and limited understanding is not recommended.
We receive our understanding of how to worship the Deity of the Lord from scripture and from our spiritual master. One must not only carefully follow the procedure of worship but also cultivate the proper consciousness of the worship. The principle is to offer some service to God with love and devotion. He appears in His deity form to receive that worship for our benefit, because without serving Him we will serve so many other lesser things and persons like movie stars, sports heroes, politicians, etc.
Krishna already has everything, but still we try to offer something, with love. When we do, He promises to accept it; just like a parent who accepts some insignificant gift from a loving child. If we follow the rules of worship under proper guidance, and we have the proper consciousness, then the Lord by His mercy receives our worship. And Krishna Himself says that one who claims to be His devotee is not His devotee, but one who is a devotee of His devotee is actually His devotee.