The idea of idolatry is the worship of anything that is not just simply the First Cause who has no form nor image. So worship of pictures of tzadikim or their graves ought to be considered idolatry as much as worship of stars. So the term עובד כוכבים עכו''ם a worshiper of stars [or worshippers of stars and constellations] does not refer to ethnic group or nationality. It is not a synonym of "gentile".

[And since this kind of worship pervades the religious world from top to bottom, I tend to avoid all of them. If there was a Litvak yeshiva in my area, I might go into that, but even then I would be uncomfortable because of the pervasiveness of idolatry anywhere and everywhere in the religious world. Certainly the Gra and Rav Shach were "מוחה" [objected], but their objections went largely unheeded.

[I ought to add that there is an obligation to be  "מוחה" [object], even if you know your objection will not be heeded as we see in the event of the concubine of Givat Binyamin in the book of Judges and the event of Kamtza and Bar Kamtza in tractate Gitin. ] 

[In fact I was hoping to study the connection between hametz and idolatry in terms of the ashes after they are burnt. But so far I have not gotten to that.]