It does not take much research at all to find out the great offense that many reformers took in the use of religious icons in worship. They viewed it as nothing less than idolatry.
Calvin dedicated a section of his Institutes to explain how this idolatry, though originally opposed by ancient church Fathers (he argues), became an established sin in the church. For example herhere Calvin establishes some history of his own to work with.
Augustine distinctly declares, that it is unlawful not only to worship images, but to dedicate them. And in this he says no more than had been long before decreed by the Libertine Council, the thirty-sixth Canon of which is, "There must be no pictures used in churches: Let nothing which is adored or worshipped be painted on walls. (Calvin's Institutes Book 1, CH 11 - IMPIETY OF ATTRIBUTING A VISIBLE FORM TO GOD. —THE SETTING UP OF IDOLS A DEFECTION FROM THE TRUE GOD)
Clearly, even from the title of the section, Calvin had a problem with the use of images in the worshipping of God.