<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def imageurl(self):
 """ Return the URL of a png image of the 2D structure """

        if self._imageurl is None:
            self._imageurl = 'http://www.chemspider.com/ImagesHandler.ashx?id=%s' % self.csid
        return self._imageurl