Hi,
yes this is a bug, thanks for report it.
To fix replace line 66 at file /administrator/components/com_jsn/models/fields/image.php from
$html[]=JHtml::_('image', $this->value ,$this->element['alt'],$attribs);
to
$html[]=JHtml::_('image', preg_replace('~_(?!.*_)~', 'mini_', $this->value) ,$this->element['alt'],$attribs);