You can see the definition of both here: idol and icon.
a devotional painting of Christ or another holy figure, typically
executed on wood and used ceremonially in the Byzantine and other
a person or thing regarded as a representative symbol or as worthy of
an image or representation of a god used as an object of worship
a person or thing that is greatly admired, loved, or revered: a soccer idol
I guess that in a Protestant point of view, anything or anyone that is taken as someone or something to be worshiped besides God is wrong, idolatry.
I had a talk with a Catholic priest about it and he assured me that they do not worship the saints, they are mediators and I showed him this:
1 Timothy 2:5-6
5 For there is one God and one mediator between God and mankind, the
man Christ Jesus, 6 who gave himself as a ransom for all people
So he changed his answer from mediators to examples, so they take the saints as examples to be remembered, respected and also an example of a good life with God.
I don't agree with the "examples" answer from that priest because you can see that the saints, according to the Catholics and Orthodox Church, they can do miracles and that, in my opinion is what makes them adore the Saint and have them as mediators. And also, because of that, you can see that there is a lot of saints and each one have one "responsibility", one saint takes care of drivers, another of doctors and so on (at least here in Brazil I see a lot of that).
So that's my opinion. Just to let you know, I'm not Catholic or Protestant so I'm not trying to take any sides here OK?