Just add the code in the indicated section of this url:
https://img.youtube.com/vi/<codigo>/0.jpg
Example

link
What I did was leave a still image. When the user clicks on the image, the iframe (without src) receives the src
attribute, which is written to data-src
of the image.
These days ago, I did something like this:
$(function(){
$('#img').click(function ()
{
var src = $(this).data('src');
$('#frame').attr({src: src});
});
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script><imgid="img" src="https://img.youtube.com/vi/zzOv14tnaWU/0.jpg"data-src="https://www.youtube.com/watch?v=zzOv14tnaWU" />
<iframe id="frame"></iframe>