The width of the email templates is less than the width of the other pages, so images or tables that show in the these templates need to be no wider than 490 pixels. If they are too wide, they will force the body of the template to be wider than the email header, making the header look too small.
If the image is in your Event Details, you will need to adjust the size there. Otherwise, change the size of the image in the email. To change the image size, simply click the image and then click the last icon that looks like an X) to change the size. Make sure it's 490px or less.
Note: Videos on email templates, even those added through event description variables, will not be sent through email. You should add a screenshot of the video to the template and then link the image to the hosted video.