When talking about responsive images we can often be talking about one of two things.

CSS styling to make an image tag resize itself visually as the browser changes sizes.
A technique for serving images of differing dimensions dependant on the size of the browser.

Often these techniques are used in unison because the first ensures image are visually the correct size whereas the second ensures that we are not wasting bandwidth by serving really large images to really small devices.

Here is a really good overview/primer explaining some of the high level concepts: https://www.lullabot.com/articles/fundamentals-of-responsive-images

CSS

The first technique is fairly straight forward and can be achieved with the following css:

img {
	display: block; /* Make sure img tag acts like a block level element. */
  	max-width: 100%; /* Part 1: Set a maximum relative to the parent. */
  	height: auto; /* Part 2: Scale the height according to the width, otherwise you get stretching. */
}

When using bootstrap this can be made even simpler by simply extending the 'img-responsive' class:

img {
	@extend .img-responsive;
}

Markup

The second technique is a lot more involved especially when we factor in content management. The following is markup generated by Drupal from a single source image:

<picture>
  <source srcset="
              /sites/default/files/styles/pod_1x/public/images/pods/clinical_team.jpg?itok=0eSda4ao 1x,                               /sites/default/files/styles/pod_2x/public/images/pods/clinical_team.jpg?itok=C2Sm4AR1 2x                            "
    media="(max-width: 479px)">
  <source srcset="
              /sites/default/files/styles/pod_1x/public/images/pods/clinical_team.jpg?itok=0eSda4ao 1x,                               /sites/default/files/styles/pod_2x/public/images/pods/clinical_team.jpg?itok=C2Sm4AR1 2x                            "
    media="(min-width: 480px)and (max-width: 767px)">
  <source srcset="
              /sites/default/files/styles/pod_1x/public/images/pods/clinical_team.jpg?itok=0eSda4ao 1x,                               /sites/default/files/styles/pod_2x/public/images/pods/clinical_team.jpg?itok=C2Sm4AR1 2x                            "
    media="(min-width: 768px) and (max-width: 991px)">
  <source srcset="
              /sites/default/files/styles/pod_1x/public/images/pods/clinical_team.jpg?itok=0eSda4ao 1x,                               /sites/default/files/styles/pod_2x/public/images/pods/clinical_team.jpg?itok=C2Sm4AR1 2x                            "
    media="(min-width: 992px) and (max-width: 1199px)">
  <source srcset="
              /sites/default/files/styles/pod_1x/public/images/pods/clinical_team.jpg?itok=0eSda4ao 1x,                               /sites/default/files/styles/pod_2x/public/images/pods/clinical_team.jpg?itok=C2Sm4AR1 2x                            "
    media="(min-width: 1200px)">
  <img src="/sites/default/files/styles/pod_2x/public/images/pods/clinical_team.jpg?itok=C2Sm4AR1" width="224" height="224">
</picture>

The picture element works similarly to the video element; a wrapping element contains multiple source elements. This allows the browser to decide which source to use under a certain set of criteria. In the case of the picture element the criteria is set by the 'media' attribute which contains a CSS media query that informs the browser under which circumstances the given source should be used.

Bootstrap

If you are using the Bootstrap grid framework then the following tool could be helpful: https://kalamuna.github.io/bootstrap-picture-srcset-generator/

Given an aspect ratio, this tool generates picture/source markup based on how many grid units wide you want your image to be at any given breakpoint. This markup can be copied directly into html templates or alternatively you can use the image widths and heights generated at each breakpoint to inform the creation of Drupal image styles and responsive images.

TODO:

Drupal config

WordPress config

Best practices

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