Adding permalink on click of image in custom excerpt

I have a website which has embedded videos in it’s posts. However, I am using screenshot images of these videos in my excerpts (hence Using the custom excerpt meta box). How do I wrap <a href> .. </a> around the image so it links to the permalink? Given that I cannot use php in the custom excerpt meta box?

If it were possible, the code in the excerpt box would be something like :
<a href="<?php the_permalink(); ?>"><img src="my-image-url.jpg"></a>

Is there a work around for this?