getUserGravatar() in JOSC_post.php will only check built in templates for nophoto.png. The slight tweak below will allow PNG and JPG nophoto's in any template folder...

~line 442...

$gravatar_email = $this->_item['email'];

		$uripng = new JURI("$this->_template_path/$this->_template_name/images/nophoto.png");

		$urijpg = new JURI("$this->_template_path/$this->_template_name/images/nophoto.jpg");

		$checkpathpng = JPATH_BASE.str_replace( '/', DS, $uripng->getPath());

		$checkpathjpg = JPATH_BASE.str_replace( '/', DS, $urijpg->getPath());

		if ( file_exists($checkpathpng) ) {

			$default = "$this->_template_path/$this->_template_name/images/nophoto.png";

		} elseif ( file_exists( $checkpathjpg ) ){

			$default = "$this->_template_path/$this->_template_name/images/nophoto.jpg";

		} else {

			$default = JURI::base().'components/com_comment/assets/images/nophoto.jpg';

		}

Hope this helps...