Q: What is forbidden in the second commandment?
A: The second commandment forbiddeth the worshipping of God by images, or any other way not appointed in his word.

Deuteronomy 4:15-19. Take ye therefore good heed unto yourselves; for ye saw no manner of similitude on the day that the LORD spake unto you in Horeb out of the midst of the fire: Lest ye corrupt yourselves, and make you a graven image, the similitude of any figure, the likeness of male or female, the likeness of any beast that is on the earth, the likeness of any winged fowl that flieth in the air, the likeness of any thing that creepeth on the ground, the likeness of any fish that is in the waters beneath the earth: and lest thou lift up thine eyes unto heaven, and when thou seest the sun, and the moon, and the stars, even all the host of heaven, shouldest be driven to worship them, and serve them, which the LORD thy God hath divided unto all nations under the whole heaven.
Acts 17:29. Forasmuch then as we are the offspring of God, we ought not to think that the Godhead is like unto gold, or silver, or stone, graven by art and man's device.
Deuteronomy 12:30-32. Take heed to thyself that thou be not snared by following them, after that they be destroyed from before thee; and that thou enquire not after their gods, saying, How did these nations serve their gods? even so will I do likewise. Thou shalt not do so unto the LORD thy God: for every abomination to the LORD, which he hateth, have they done unto their gods; for even their sons and their daughters they have burnt in the fire to their gods. What thing soever I command you, observe to do it: thou shalt not add thereto, nor diminish from it.

Westminster Project - See Matthew Henry's Scripture Catechism based on the WSC