To make images resize responsively to page width, you can add the class
responsive-img to your image tag.
To make images appear circular, simply add
class="circle" to them
To make images center to page width, you can add the class
center-block to your image tag.
To make your embeds responsive, merely wrap them with a containing div which has the class
To make your HTML5 Videos responsive just add the class
responsive-video to the video tag.