I would like to know, for password protected videos from Vimeo, embedded on a website of mine, if there is any way to display the thumbnail of the video in the background of the password request registration.

Something like this:

Whereas currently it looks like this:

The videos and Vimeo account are mine, as is the site where I integrate them. But since the Vimeos are embedded via iframe, I have no way (or so it seems) of modifying them via CSS.

Here is the integration code on my website:

<div class="bloc-BA">
    <span class="titre-film-BA">L'Arbre des Noëls</span>
    <br>
    <span class="real-BA">Documentaire</span>
    <br><br>
    <iframe src=";byline=0&portrait=0" width="100%"
        frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>
    <br>
</div>

I wouldn't mind a solution requiring me to manually assign the right thumbnail for each video, it's just that I can't see how to do it right now. Any ideas?

(Vimeo's AI support tells me this isn't an option they offer)