Say you have a responsive website layout where images are sized at 100% of the container, but the container is not always as wide as the viewport. Do we need to use the picture tag, or the img tag is enough? The img tag can do it, using its sizes and srcset attributes. Here’s how.

The images layout

The images in the layout are wide as follows:

Viewport Width Images width Container width
0 to 767 px 1/2 of container 100% with no padding
From 768 px to 1023 px 1/3 of container 100% with 30 px padding
From 1024 px to 1279 px 1/4 of container 100% with 30 px padding
From 1280 px up 1/4 of container 1280 px with 30 px padding

Units we can use in the sizes attribute

When using the img tag, in the sizes attribute, you can specify how wide your images will display at different media queries, not only using the viewport width (vw) unit (as you could find googling around) but using any length and also using CSS calc() if you have to deal with paddings or container’s max-width.

Viewport Width Images width Container width Image width in CSS
0 to 767 px 1/2 of container 100% with no padding 50vw
From 768 px to 1023 px 1/3 of container 100% with 30 px padding calc(( 100vw - 60 px ) / 3)
From 1024 px to 1279 px 1/4 of container 100% with 30 px padding calc(( 100vw - 60 px ) / 4)
From 1280 px up 1/4 of container 1280 px with 30 px padding 305 px

See this pen which uses media queries and CSS to specify img widths, and this pen which specifies the sizes inside the img tag.

Confused? If you need a more extensive explication, please read srcset and sizes by Eric Portis, in my opinion the best article about using img with srcset and sizes

Calculating optimized image sizes based on spacing and device / orientation

We want our images to be optimized at most common resolutions, we need to:

  1. Define which are the most common screen resolutions / densities
  2. Decide which are the resolutions / densities that we want to optimize for
  3. Calculate what the image sizes will be at these resolutions / densities
  4. Scale our images to those image sizes
  5. Markup the imgs in our HTML
    1. List all the image sizes in the srcset attribute, using the w descriptor
    2. List all the image widths in the sizes attribute, as defined in the table above

To define the screen resolutions and decide which ones to optimize for, we can use any tool like Google Analytics to get some information about our users and the devices they mostly use.

After some calculations, the resulting image sizes were the following:

Device & orientation Screen width Img width (css px) Img height (css px) Screen density Img width (px) Img height (px)
iPhone 4/5/5s 320 160 186 2 320 372
iPhone 6 375 187 217 2 374 435
iPhone 5/5s landscape 568 284 330 2 568 660
iPhone 6 landscape 667 333 387 2 666 774
iPad / mini 768 236 274 2 472 549
iPad / mini landscape 1024 241 280 2 482 560
PC with 1280w up 1280 305 355 1 305 355
PC with 1280w and HiDPI 1280 305 355 2 610 709
PC with more than 1280w 1920 305 355 1 305 355

To the code!

The code of the POC that I created is on GitHub in this repo, and the live demo is here.

picture markup

<picture>
    <source media="(max-width: 320px)"
            srcset="http://placehold.it/320x372 2x">
    <source media="(max-width: 375px)"
            srcset="http://placehold.it/374x435 2x">
    <source media="(max-width: 568px)"
            srcset="http://placehold.it/568x660 2x">
    <source media="(max-width: 667px)"
            srcset="http://placehold.it/666x774 2x">
    <source media="(max-width: 768px)"
            srcset="http://placehold.it/472x549 2x">
    <source media="(max-width: 1024px)"
            srcset="http://placehold.it/241x280 1x, http://placehold.it/482x560 2x">
    <source media="(min-width: 1280px)"
            srcset="http://placehold.it/305x355 1x, http://placehold.it/610x709 2x">
    <img src="http://placehold.it/305x355" alt="A product image">
</picture>

img markup

<img src="http://placehold.it/305x355"
         srcset="http://placehold.it/241x280 241w,
             http://placehold.it/305x355 305w,
             http://placehold.it/320x372 320w,
             http://placehold.it/374x435 374w,
             http://placehold.it/472x549 472w,
             http://placehold.it/482x560 482w,
             http://placehold.it/568x660 578w,
             http://placehold.it/610x709 610w,
             http://placehold.it/666x774 666w"
         sizes="(min-width: 1280px) 305px,
            (min-width: 1024px) calc((100vw - 60px) / 4),
            (min-width: 768px) calc((100vw - 60px) / 3),
            50vw"
         alt="A product image">

Conclusion

  • picture markup turns out to be more verbose, and we’re still not supporting single density displays under 768px.
  • img markup is shorter and less repetitive, and lets the browser calculate what is the better image to use considering resolution AND pixel density.

You don’t need the picture tag.

The reason is that using the img you’ll have to write much less code to support much more devices and screen densities. Given the ability to use CSS’ calc() function in the length expression of the sizes attribute, we can do even complex calculations to define the right image to use even in complex layouts.

Note that the picture and img tags are comparable only when all the images have the same ratio over multiple media queries. If the images ratio change to adapt images to devices ratios, the picture tag is the only way to go.

Cheers!