James White (Alpha & Omega Ministries http://www.aomin.org/):
True worship must worship God as He exists, not as we wish Him to be. The essence of idolatry is the making of images of God. An image is a shadow, a false representation. We may not bow before a statue or figure, but if we make an image of God in our mind that is not in accord with God’s revelation of Himself, then we are not worshiping in truth. Since sin and rebellion are always pushing us toward false gods and away from the true God, we must seek every day to conform our thinking and our worship to God’s straight-edge standard of truth, revealed so wonderfully in Scripture. We must be willing to love God as He is, and that includes every aspect of His being that might, due to our fallen state, be offensive to us, or beyond our limited capacities to fully comprehend. God is not to be edited to fit our ideas and preconceptions.
(The Forgotten Trinity, p. 18)
If we love Him and worship Him as He deserves, we will not dare to “edit” Him to fit our desires. Instead, we will seek to worship Him in truth.
(Ibid., p. 20)
Uniqueness. Otherness. It is part of the meaning of the word “holy” itself, and God makes it plainly known that He is holy. No images, no likenesses of Him are to be allowed, for such would create a connection that does not exist. He is Creator, everything else is created. He is infinite, everything else is finite. God asks the questions of anyone who would compare Him to anything in the created order:
Who has directed the Spirit of the Lord, or as His counselor has informed Him? With whom did He consult and who gave Him understanding? And who taught Him in the path of justice and taught Him knowledge and informed Him of the way of understanding? Behold, the nations are like a drop from a bucket, and are regarded as a speck of dust on the scales; behold, He lifts up the islands like fine dust. Even Lebanon is not enough to burn, nor its beasts enough for a burnt offering. All the nations are as nothing before Him, they are regarded by Him as less than nothing and meaningless. To whom then will you liken God? Or what likeness will you compare with Him?
Source: http://www.puritanboard.com/showthread.php/81581-Reformed-Baptists-Against-Making-Images-of-the-Lord, Comment #1