Pimcore\Image\Adapter\GD::setBackgroundImage PHP Method

    public function setBackgroundImage($image, $mode = null)
    {
        $this->preModify();
        $image = ltrim($image, "/");
        $image = PIMCORE_DOCUMENT_ROOT . "/" . $image;
        if (is_file($image)) {
            $backgroundImage = imagecreatefromstring(file_get_contents($image));
            list($backgroundImageWidth, $backgroundImageHeight) = getimagesize($image);
            if ($mode == "cropTopLeft") {
                $newImg = $this->createImage($this->getWidth(), $this->getHeight());
                imagecopyresampled($newImg, $backgroundImage, 0, 0, 0, 0, $this->getWidth(), $this->getHeight(), $this->getWidth(), $this->getHeight());
                imagealphablending($newImg, true);
                imagecopyresampled($newImg, $this->resource, 0, 0, 0, 0, $this->getWidth(), $this->getHeight(), $this->getWidth(), $this->getHeight());
                $this->resource = $newImg;
            } else {
                // default behavior (fit)
                $newImg = $this->createImage($this->getWidth(), $this->getHeight());
                imagecopyresampled($newImg, $backgroundImage, 0, 0, 0, 0, $this->getWidth(), $this->getHeight(), $backgroundImageWidth, $backgroundImageHeight);
                imagealphablending($newImg, true);
                imagecopyresampled($newImg, $this->resource, 0, 0, 0, 0, $this->getWidth(), $this->getHeight(), $this->getWidth(), $this->getHeight());
                $this->resource = $newImg;
            }
        }
        $this->postModify();
        return $this;
    }