The idealist position would have to argue there is no original source (no hypokeimenon). But according to Epicurus, if there is no original source, the object is not real -- and the proposition that it is real, is necessarily false (such as the proposition "minotaurs exist").
This aspect of the "idealist" position as cited by Bryan reminds me to clarify: I think Epicurus' position on divinity served important goal-identifying and psychological purposes for Epicurus (which is a positive use of the term ideal), just as reverencing the sage is of great benefit to him who does the reverencing). IN ADDITION I think Epicurus thought such beings of such a class really exist in the universe, both because we receive "images" of them and because it makes sense that such beings do exist in an infinite and eternal universe. I think these "real" and "ideal" aspects go hand and hand and there's no conflict between them. To the extent that those that hold the "idealist" position also hold that these gods do *not* exist in reality, but solely as mental constructs, I don't agree with that aspect of the "idealist" view. Sorry to interrupt the flow of the discussion but this is for any lurkers who might be confused by my earlier comment.