Wow - for some reason i thought it was about a FORM on a landing page. If you're talking about getting images / Videos in responsive emails. The best thing to do would be to set a max-width="400px" / class="max-width: 400px;" (or whatever) Rather then declaring a set pixle width. That should make them size down for you. OR you could use
@media Query for your classes that will size it down per screen size. There's a few ways to do it. Although when it comes to video. I would embed a link from youtube or something for that. Video in email gets a little tricky with email clients. I normaly take a screen shot / design a "cover image" for the video. Just for a clear CTA that looks good.
Here's a list of Marketos responsive tempaltes. Might be a good place to start
http://templates.marketo.com/category/responsive-email/