"Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath; or that is in the water under the earth; Thou shalt not bow down thyself to them, nor serve them, for I the Lord thy God am a jealous God."

The duties required in the second commandment are the receiving, observing, and keeping pure and entire, all such religious worship and ordinances as God has instituted in His Word; particularly prayer and thanksgiving in the name of Christ; the reading, preaching, and hearing of the Word; the administration and receiving of the Sacraments... (taken from the Larger Catechism of the Presbyterian Church - Answer to Question 108)

The sins forbidden in the second commandant are all devising, counseling, commanding, using, and any wise approving any religious worship not instituted by God himself; the making any representation of God, of all or of any of the three Persons, either inwardly in our mind or outwardly in any kind of image of likeness of any creature whatsoever: all worshiping of it, or God in it or by it;...corrupting the worship of God, adding to it, or taking from it, whether invented and taken up of ourselves or received by tradition from others,...all neglect, contempt, hindering and opposing the worship and ordinances which God has appointed. (taken from the Larger Catechism of the Presbyterian Church - Answer to Question 109)
Forbidden is the worshiping of God in the manner which pleases self - you own will, intellect, or emotions, and/or heeding the traditions of man apart from the Word of God.