I'm trying to convert some pdf files to jpg through Wand in Python:

```
from wand.image import Image as Img
from wand.color import Color

    def importPdf(self):
        filename, _ = QtWidgets.QFileDialog.getOpenFileName(self, "Open File",
                                                            QtCore.QDir.currentPath())
        print(filename)
        if not filename:
            print('error')
            return
        with Img(filename=filename,format='jpeg', resolution=300) as image:
            image.compression_quality = 99
            image.save(filename='file.jpeg')
            self.open_picture()

```

My problem is that it results is a black screeen. The conversion works fine with png, but I cannot perform the OCR (via tesseract on the png).
I think it comes from a kind of transparent layer, but I have not found the way to remove it, though I did several things such as 

```
image.alpha_channel = False # made the same with True
image.background_color = Color('White')

```

before saving the file.
I'm using Imagemagick V6.9, because V7 fails with Wand.