You can change the content of an iframe via <a>
links with a target attribute with a value matching the name attribute of the frame.
So the iframe has a name:-
<iframe src="placeholder.jpg" name="frame" height="266" width="266" class="window"></iframe>
And the link to switch content has the iframe as target and the new content as its href:-
<a href="frame-content.mp4" target="frame">Load Content</a>
The iframe will initially show the placeholder image. When the link is clicked it will show the frame-content media.