When the people of the exodus had Aaron make the golden calf, it was so that they could worship it, pray to it, and sacrifice to it as a focus of whatever religion it represented. Gos's command for things like the cherubim on the ark was to remind His followers of the sacred nature of the contents of the ark. In the same way, icons are to be a reminder of either the story depicted, or of the person depicted as a kind of visual record in the same vein as a written record. Unfortunately, there have always been those who shift the focus to the icon as a power in its own right, such as being able to heal the sick, etc., much like the relics of the Middle Ages, which were considered to be totems with powers in their own right.
I know I haven't expressed this well, but to me the big difference is that such objects are to be reminders of God, not facilitators in their own right. That is what idolatry is, treating the idol as something empowered to do acts in its own right.