We noted in our commentary on WLC 103, that the 1st commandment has to do with the proper object of worship; whereas the 2nd has to do with the proper manner of worship. Therefore, it is an error to speak of idolatry proper (i.e. all worship involving idols and images) as being condemned only in the 2nd commandment rather than the first. But this is a subtle error of modern evangelicalism and fundamentalism. The error of Rome is to combine the 1st and 2nd commandments into one so that there appears to be no explicit commandment on how God ought to be worship. So it is said that whatever is not forbidden is allowed in the worship of God. So images are allowed in the worship (or they say, ‘service’) of God because Exodus 20:4-6 is (according to them) part of the 1st commandment and forbids the use of images to worship other gods only.(more at the link above)
Modern evangelicalism and fundamentalism agrees that Exodus 20:4-6 is the 2nd Commandment. But because it regards this commandment as forbidding idolatry proper only, it tends to fail to see that this commandment is not just about the use of images, but is, in fact, about how the Living and True God ought to be worship. That is to say: Though the 2nd Commandment refers to the use of images, it refers to it only by way of a concrete example of how the worship of God is commonly violated. It is commonly, (indeed almost universally) assumed that God may be better worship if there is a visual representation of Him. God is saying that is wrong. He is saying that by the 2nd Commandment that we may not worship Him according to the manner, which we imagine to be helpful or right. We must worship Him only according to His own appointment. Whatever mode of worship which is not sanctioned in His Word is idolatrous (in the wider sense of the word) and sinful.
(more at the link above)
The 2nd Commandment has to do with the proper manner of worship. It is not only about the use of images, for all worship that is not in accordance to biblical prescription is idolatrous worship whether or not images are used. The duties required in the 2nd Commandment, therefore, pertain to the practice and defence of biblical worship.
What is biblical worship as opposed to idolatrous worship? We may roughly define biblical worship as a formal activity of fellowship between God and His children in which,—other than attending circumstances (such as pertaining to physical arrangement, location and time),—all distinguishable elements (such as prayer, preaching, singing of psalms, and the administration of sacrament), are warranted by the Scriptures. Conversely, a worship event in which there are distinguishable elements which find no warrant in the Scripture (such as flag waving, clap-offering, candle light procession, puppet show, guitar rendition, piano introduction, etc) must be regarded as unbiblical and idolatrous worship.
(more at the link above)
The 2nd Commandment, as we saw earlier, is not only about making of idols and images. Rather, it is about the acceptable manner of worship of the living and true God. Thus it does not only forbid crafting of idols of wood and stone, and bowing down to them in worship. It forbids the abuse of true worship and abominates all forms and elements of religious worship "not instituted by God Himself" in His Word.
Our answer summarises the deeds forbidden under nine categories.
First, it forbids "all devising, counseling, commanding, using, and any wise approving, any religious worship not instituted by God Himself." That is, it forbids any positive attitude and relationship towards worship not appointed by God Himself. When a minister, for example, introduces an unbiblical song into a worship service, he breaks the 2nd Commandment by leading the congregation to use what God has not sanctioned for use in His worship, and the congregation break the same commandment by using the song.
Secondly, the 2nd Commandment forbids "tolerating a false religion." Take careful note this does not mean at all that Christians should persecute those who hold or practice false religion. Neither does it mean that we should follow what Jenny Geddes did on 23 July 1637 when she flung her prayer stool at the preacher in St Giles’ Cathedral, Edinburgh, who was trying to read from an idolatrous prayer book introduced by Archbishop Laud. But it does mean that we should not sit idly and do nothing when false religion is being introduced into the church whether by the leaders or by members (cf. Deut 13:6-11).
Thirdly, the 2nd Commandment explicitly forbids "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 or likeness of any creature whatsoever" (cf. Ex 20:4). This means that Michelangelo broke the 2nd Commandment when he painted "The creation of Adam," in 1511. Believer ought not to use that picture, however famous it may be.…
And not only so, but the use of pictures of Christ is also forbidden in the 2nd Commandment, and so is any attempt to imagine how He looks like or to picture him in the mind.
Some may argue that the artistic or educational use of pictures of God is not forbidden in the 2nd Commandment since they are not used for worship, but the fact remains that they are religious pictures and they tend to promote mental idolatry even if they are not intended originally for worship. Therefore all pictorial representations of God are rightly forbidden under the 2nd Commandment.
But what about pictures of man and animals? What about photographs? Exodus 20:4 seem to forbid their use when it says: "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" (Ex 20:4). A proper understanding of the commandment however shows that this is not the case. Our catechism, as such, rightly points out that what is forbidden is not their use per se, but their use to represent God.
(more at the link above)
God spoke the Ten Commandments in the hearing of His people. There is an apparent order in the giving of the commandments, which becomes clear especially when we come to the second table, for we naturally understand that murder is worse than adultery, which is worse than stealing, which is worse than coveting. But it is also apparent that there is a different degree of emphasis in the giving of each of the commandments. In particular, it is quite obvious that God took much greater pains to explain and emphasise the 2nd and the 4th Commandments. It may not be immediately clear why God does not emphasise the 1st Commandment more than these two commandments, but once we realise that the 1st Commandment distinguishes God’s people from the rest of the world, and how God’s people is easily tempted to break the 2nd and 4th Commandments, we can easily deduce why God in His wisdom had chosen to emphasise and clarify them in such a way. For, is it not true that even today, the two commandments that is most neglected, if not opposed in visible Christendom are these two commandments? After all, how many churches today would still worship according to the biblical principle of worship that whatever is not sanctioned is forbidden; and how many churches today believe that Christians must still diligently keep Sabbath?
With this in mind, let us consider the annex to the 2nd Commandment which not only give it emphasis, but serves ‘the more to enforce it.’
Also, there are more comments on other WLC questions at this link.