function withImage() return false

I am using this package to apply watermark on pdf files bu watermark does not merged with pdf file, instead **withImage()** function return **false**.

```
$watermark = 'images/watermark.png';

if ($model->extension == 'pdf') {
$pdf = new Watermark($fileName); // $fileName is string that contain path to pdf file
$pdf->withImage($watermark);
}

```

The problem is, it does not throw any error like file not exist or etc. it just returns false.