Daniel Matthew

Implementing Responsive Images

Updated 26/06/2015: The sizes attribute is no longer optional if you want your code to validate.

I'm hoping that you'll notice the site - particularly my work section - is feeling a little snappier today: that's because I took it upon myself to implement a solution for responsive images.

Take a look at this screenshot (also a responsive image, fact-fans): Before: Screenshot showing the network tab within Chrome's Developer Tools

Chrome's Developer Tools are telling us that a request to /work had the browser download 3.4 MB over 13.77 seconds - on a fast ADSL connection. That's pretty bad news for a visitor using a mobile phone on a cellular connection: whether it be the time needed for the page to finish loading, or if their provider imposes a data cap.

I have a confession: I was being pretty lazy. The images presented were simply screencapped in OS X and uploaded. I didn't resize them or convert them from PNG-24 to a more pallatable format for the web. It's not like I needed the transparency! In fact, I could probably go further: I could crop out the drop-shadow and browser chrome, and implement those additions with CSS.

There are a few options open to a developer wishing to implement responsive images:

The srcset attribute

srcset allows the developer to provide the browser with multiple resolutions of the same image.

<img srcset="/public/img/build/ribbonworks-new-full.jpg 1307w,
/public/img/build/ribbonworks-new-med.jpg 654w,
/public/img/build/ribbonworks-new-small.jpg 327w"
alt="Ribbonworks.co.uk" />

The srcset attribute is simply a comma-separated list of URIs and a "width descriptor", which is used by the browser as part of its decision-making process.

The sizes attribute

When I first wrote this entry, I'd neglected to mention this attribute as I hadn't considered it to be a major requirement. After all, with my single-column layout I'm happy for my images to always fill the available space.

But what if you're not? Try this:

<img srcset="/public/img/build/ribbonworks-new-full.jpg 1307w,
/public/img/build/ribbonworks-new-med.jpg 654w,
/public/img/build/ribbonworks-new-small.jpg 327w"
sizes="(max-width: 30em) 100vw, (max-width: 48em) 50vw, 33vw"
alt="Ribbonworks.co.uk" />

The sizes attribute tells the browser what portion of the viewport the author intends the image to assume at a given breakpoint. This is intended to prevent larger images being downloaded unnecessarily. By default - and why I've gotten away with it - it assumes the image will be 100vw.

In the above example, a viewport up to 30em in width will download the image that will fill the available space. After that, it'll do the maths to figure out which image to display between 30 and 48em. And once again once that breakpoint has been passed.

The picture element

The new picture element in HTML allows the developer to implement 'art-direction': they can choose the image displayed at a particular breakpoint. My screenshots are all of a desktop browser. I could use the picture element to display these images as they appear on mobile, or tablet browsers. The syntax is a slight departure from using img:

  <source media="(max-width: 36em)" srcset="/public/img/build/ribbonworks-new-mobile-full.jpg 1307w,
/public/img/build/ribbonworks-new-mobile-med.jpg 654w,
/public/img/build/ribbonworks-new-mobile-small.jpg 327w" />
  <source srcset="/public/img/build/ribbonworks-new-full.jpg 1307w,
/public/img/build/ribbonworks-new-med.jpg 654w,
/public/img/build/ribbonworks-new-small.jpg 327w" />
  <img src="/public/img/build/ribbonworks-new-full.jpg" />

The developer provides source elements and an img. The browser evaluates the sources until the parameters provided in media attribute are true. The srcset from this source is then applied to the img.

Processing Images

The main obstable to implementing this isn't browser support (there are multiple quality polyfills, such as Picturefill), but actually creating multiple versions of each of your images. I ended up using Photoshop and saving each file out at 50 and 25 percent. In doing so, I encountered its "Extract Assets" feature which would make the process a cinch - if its output matched my workflow. I'd looked at ways of automating it, and found several Grunt plugins, including grunt-responsive-images, though I got turned off at the need to install Imagemagick. Adding this functionality to a platform such as WordPress would be seemingly trivial - again, so long as a module such as Imagemagick is installed. WordPress already has the ability to process images and create multiple sizes for different roles.


Simply converting my portfolio images to JPEG reduced the required bandwidth down to 1.5 MB: a 42% reduction. As Kevin Bacon would say: "a no-brainer". By taking advantage of srcset and providing scaled-down versions of the images, the site loaded in just 2.66 seconds, and only transferred 522 KB: After: Screenshot showing the network tab within Chrome's Developer Tools

Worthwhile reductions methinks, and this just on a small site. If I were serving many thousands of impressions a day, I know that this would be one way to keep my bandwidth costs down, alongside making the browsing experience more tolerable for everyone.

Daniel is a front-end developer at Talis, with an interest in accessibility, performance, and web components.