Well, then in your case it will look like:

private function _getUnitThumbUrl($sSize, $aData, $bSubstituteNoImage = true)

{

$CNF = &$this->_oConfig->CNF;

    $sMethod = 'url' . bx_gen_method_name(str_replace('ava', 'avatar', $sSize), array('_', '-'));

    $sUrl = '';

    if(method_exists($this, $sMethod))

        $sUrl = $this->$sMethod($aData, $bSubstituteNoImage);

    else

        $sUrl = $this->_getUnitThumbUrl($this->_sUnitSizeDefault, $aData, $bSubstituteNoImage);

    if (strpos($sUrl, 'no-picture-') !== false)

    {

        $aData[$CNF['FIELD_ID']] = 10; // or any other necessary id

        $sUrl = $this->_getUnitThumbUrl($sSize, $aData, $bSubstituteNoImage);

    }

    return $sUrl;

}