video with alpha channel : black thumbnail...

Good evening everyone.
I am relatively new to the site, but I have the following problem:
I basically create black silhouettes on a transparent background. I encode them in mov + png codec.
Unfortunately, on the miniatures of research, the background is black .... So it makes a beautiful black rectangle.
Some thumbnails of videos with alpha channel have a white background and I want advice from those who have this result.
Thanks and good videos to all.