// this will show image with white background:
$thumb = new GD($imgPath);
$thumb->resize(40, 40);
$thumb->pad(40, 40, [255, 255, 255]);
$thumb->save($thumbFilePath);

// this (I WOULD LIKE) will show image with transparent background:
$thumb = new GD($imgPath);
$thumb->resize(40, 40);
$thumb->pad(40, 40, false); // or any other call format to specify we wont transparent background
$thumb->save($thumbFilePath);

// near line 147
        if (!$color) {
            imagealphablending($this->workingImage, false);
            $fillColor = imagecolorallocatealpha($this->workingImage, 0, 0, 0, 127);
            imagefill($this->workingImage, 0, 0, $fillColor);
            imagesavealpha($this->workingImage, true);
        } else {
            // create the fill color
            $fillColor = imagecolorallocate(
                            $this->workingImage, $color[0], $color[1], $color[2]
            );

            // fill our working image with the fill color
            imagefill(
                            $this->workingImage, 0, 0, $fillColor
            );
        }