Thanks for your feedback!

At first I just enter the link to “Project Image”

Now I tried this embed code in the field “Project Description”
<iframe width="360" height="200" src="https://www.youtube.com/embed/wW-5nQWl0tk" frameborder="0" allowfullscreen></iframe>

It works well but isn’t really responsive like the other pictures.
This class to the iframe doesn’t bring the desired results.
alt class="img-responsive center-block"

Have you another idea to make the video responsive?