<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def getPageImageList(self, pno):
 """Retrieve a list of images used on a page. """

        if self.isClosed or self.isEncrypted:
            raise ValueError("operation illegal for closed / encrypted doc")
        if self.isPDF:
            return self._getPageInfo(pno, 2)
        return []