Idol worship means placing anything in our life above God. If you make an idol and consider it as God, then you are doing idol worship. If you place money above God in life, you are an idol worshiper. If you place a woman or man above God in your life, then you are an idol worshiper.
The statues or pictures used in churches are just for the spiritual enhancement of believers. They know that those are pictures and idols. They are not worshiping that idol or picture, but him, whom it represents. An idol in a church is just like a ladder. You can climb up easily to a tree or to top of your roof with the help of a ladder. Same way, in churches, pictures or idols are just used as ladders to reach to God.