<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _gather_image_parts(self):
 """Load the image part collection with all the image parts in package."""

        for rel in self.iter_rels():
            if rel.is_external:
                continue
            if rel.reltype != RT.IMAGE:
                continue
            if rel.target_part in self.image_parts:
                continue
            self.image_parts.append(rel.target_part)