iframe video no thumb image

Hi,

I loaded a video in my project and display it on my web page.

<iframe width="560" height="315" src="Videos/myvideo.mp4" frameborder="0" allowfullscreen></iframe>

The video can be played, but the thumb appears a black box. I tried to use an image with z-index: 10. but it did not work.

Could someone tell me how to load an image as thumbnail with the video?

Thank you,

Ella W