פירוש על עבודה זרה 81:10

One who finds a fragment of an image may make use of that fragment. The mishnah teaches that we may assume that the non-Jew intentionally broke the idol and thereby annulled it from its idolatrous use.
However, if one finds an entire hand of an image or a foot, it is prohibited, since there are non-Jews who use these parts in and of themselves as idols.