don't try to save all the frames when saving to a format that doesn't…

… support that

libweasyl/libweasyl/images.py

@@ -118,7 +118,7 @@ def _write(self, out, *args, **kwargs):
         kwargs['format'] = kwargs['format'].lower()
         if kwargs['format'] == 'jpg':
             kwargs['format'] = 'jpeg'
-        if self.is_animated:
+        if self.is_animated and kwargs['format'] in ['gif', 'webp']:
             kwargs['save_all'] = True
             kwargs['append_images'] = self._frames[1:]
             kwargs['duration'] = self.duration