Hello the video player does not play on mobile devices you have to provide a poster image for them.

Hello, absolutely

you can change the poster image in Appearance -> Theme options -> Home Options

You set everything correctly. Can you provide wp admin access in a private comment so I can have a look to see what's wrong?

Hi can you check now? Also clear your cache.

Go to style.css and then change this bit

.video-section {
          background-image: url('http://invideo.fr/wp-content/uploads/2017/05/cropped-cropped-nitray9-4.jpg') !important;
    }
@media only screen and (max-width: 1024px) {
    
    .video-overlay {
        background-size:cover;
        background-position:center center;
        width:100%;
        height: 100%;
        background-image:url('http://invideo.fr/wp-content/uploads/2017/05/cropped-cropped-nitray9-4.jpg');
    }
    
}

replacing the url of the image