Yes, you may embed images and videos.

Remarkbox does not host images or videos so they would need to be "hotlinked" or hosted on another web server or service and embedded into the comment.

For example - I can hotlink an image hosted by LinkPeek like this:

<img src="https://linkpeek.com/api/v1?uri=www.remarkbox.com&apikey=9fhvyH9KP&token=87edfd1ee925cf4c1b9de89b7d8b7cd2&size=480x340&viewport=1024"/>

And the result is this: