1) Under email builder, go to promotional info.
2) Drag and drop the image to the promotional info section.
3) Double-click the image to go to image properties. In here, you can change the size of the image(set the height from 300-350), as well as change the alignment(center).
4) Once, alignment and image size is set. Click on the image and select the hyperlink tool.
5) On the URL field, enter the youtube video link.
6) Click OK to Save the Changes.