You could use media queries in your CSS to target the screen size and then amend the image/video sizes. If you want to have everything 100% you could add the bootstrap class img-responsive.
/* CSS for mobile screens (up to 768px) */
@media only screen and (max-width: 768px) {
img { width:100%; height:auto; }
.or-use-image-class { width:100%; height:auto; }
iframe { width:100%; height:auto;}
.or-use.video-class { width:100%; height:auto; }
}