how to phpthumb gd_watermark watermark dynamically scale inside orginal image

this code worked but not scaling watermark. if small image then very big watermark, if big image then very small watermark . so this my code

 public function watermark($fullPath)
        {

            if(StoreImagesConfig::get('watermark_active'))
            {
                $pic = PhpThumbFactory::create($fullPath);
                $pos = StoreImagesConfig::get('watermark_position_vertical').StoreImagesConfig::get('watermark_position_horizontal');

                try {
                    $watermark = PhpThumbFactory::create(Yii::getPathOfAlias('webroot.uploads') . '/watermark.png');

                    $pic->addWatermark(
                        $watermark,
                        $pos,
                        StoreImagesConfig::get('watermark_opacity'),
                        0,
                        0
                    );
                    $pic->save($fullPath);
                } catch(Exception $e) {
                    // pass
                }
            }
        }       

gd_watermark.php addWatermark function

public function addWatermark($wm, $pos, $opacity, $offsetX, $offsetY, $that)
    {
        $picDim = $that->getCurrentDimensions();
        $wmDim = $wm->getCurrentDimensions();

        $wmPosX = $offsetX;
        $wmPosY = $offsetY;

        if(preg_match('/right|east/i', $pos)){$wmPosX += $picDim['width'] - $wmDim['width'];}
        else if(!preg_match('/left|west/i', $pos)){$wmPosX += intval($picDim['width']/2 - $wmDim['width']/2);}

        if(preg_match('/bottom|lower|south/i', $pos)){$wmPosY += $picDim['height'] - $wmDim['height'];}
        else if(!preg_match('/upper|top|north/i', $pos)){$wmPosY += intval($picDim['height']/2 - $wmDim['height']/2);}

        $workingImage = $that->getOldImage();

        $wmImage = ($wm->getWorkingImage() ? $wm->getWorkingImage() : $wm->getOldImage());

        $this->imageCopyMergeAlpha($workingImage, $wmImage, $wmPosX, $wmPosY, 0, 0, $wmDim['width'], $wmDim['height'], $opacity);

        $that->setWorkingImage($workingImage);
        return $that;
    }

how to resize watermark inside orginal image....