His issue in this section of the Institutes is NOT to deal in particular with saint worship (though it's related, since many of the statues used by the Roman church at the time and today are of the saints, and which do receive idolatrous worship), but with the use of images to represent God as an aid to worship. He calls the practice idolatry, and brings up the case of Aaron and the Israelites in the wilderness, saying that the Jews
"weren't so thoughtless as to forget that it was God by whose hand they had been led out of Egypt [Lev. 26:13] before they fashioned the calf [Ex. 32:4]. But when Aaron said that those were the gods by which they had been set free from the land of Egypt, they boldly assented [Ex. 32:4,8], obviously meaning that they wished to retain that liberating God, provided they could see him going before them in the calf. And we must not think the heathen so stupid that they did not understand God to be something other than stocks and stones. For while they changed images at pleasure, they always kept the same gods in mind. (pp. 109-110, Institutes of the Christian Religion)The heart of the matter, again, is this - to use images for the purpose of worship - exciting worshipful attitudes, praising God by using those images to bring to mind the reality of God Himself, etc - is the kind of activity the second commandment forbids. It is NOT helpful in the least, Calvin rightly argues, but leads one into false understandings of God, and into false worship of Him. God has not any form whatsoever - and Scripture gives us no description of Christ Himself, that (I believe) we might not be led into fashioning images of Christ, either, for the purpose of worship. The heart of the commandment, again, is not that we worship false gods in these images. That is a violation of the first commandment, a taking of other gods. The second deals with physical representations, and the fact that God commands that we worship Him in spirit (no images) and truth (no image can represent Him properly - or even represent Jesus Christ properly, since in order to properly represent Jesus Christ, we must not simply represent His physical body - for that would not be a true representation of Him as God-Man).
Among the arguments the Roman Church used in Calvin's day was that these images were supposed to be the "books for the laity" - for the ignorant, who couldn't read the Scriptures. Of course it didn't help those ignorant that the clerics kept the Word of God from them by insisting that it appear only in the ecclesiastical tongue, Latin, and then only in the possession of the priests. Thus, since they couldn't understand God's Word, the priests argued, they must have images. A whole industry was constructed around the "need" (artificially created) for images that served only to keep the clerics in power, and suppress the ability of the laity to know and serve God according to His will. Calvin argues forcefully that the existence of the ignorant masses is entirely of the doing of the priests... and he argues that by them, the priests made idolaters of many. Thank God for John Wycliffe, William Tyndale and Martin Luther, who, among others, insisted on the importance of the Word being understandable to all, that we might all be instructed in the truth, and not kept in the dark, confused and distracted by images!
To return to the heart of the matter. Calvin, in section 9, refers back to Augustine, 1100 years prior to Calvin's day, speaking of images, and the excuses offered even then, to the complaint that images are impermissible as aids to worship. Here in closing is the relevant section, for your consideration:
Read the excuses that Augustine refers to as having been pretended by the idolaters of his own age: when they were accused, the vulgar sort replied that they were not worshiping that visible object but a presence that dwelt there invisibly. Those who were of what he called “purer religion” stated that they were worshiping neither the likeness nor the spirit; but that through the physical image they gazed upon the sign of the thing that they ought to worship. What then? All idolaters, whether Jews or pagans, bwere motivated just as has been said. Not content with spiritual understanding, they thought that through the images a surer and closer understanding would be impressed upon them. Once this perverse imitation of God pleased them, they never stopped until, deluded by new tricks, they presently supposed that God manifested his power in images. In these images, nevertheless, the Jews were convinced that they were worshiping the eternal God, the one true Lord of heaven and earth; the pagans, that they were worshiping their gods whom, though false, they imagined as dwelling in heaven. (pp. 110, Institutes of the Christian Religion)