The Smart Shiba's Guide to Images
​on the Web

by Jessica Parsons, with help from Monty and Petunia

Why does it matter?

  • Images are ~60% of web content (sources: 1, 2, 3, 4)
  • Fast hosting & CDN gets you partly there—reducing file size finishes the job
  • Must balance file size (speed) vs quality

Photo credits:

All images are my own unless otherwise specified.

Areas for Improvement

  • Better image files
  • Code for responsiveness
  • Server settings
  • Workflow & services

File-Level Improvements

  • Choose the right format
  • Optimize compression
  • Choose the right size

Choosing the Right Format

  • Depends on image content
  • Features vary
  • Balance file size vs. quality
  • Newer formats change the game

Photos ➔ JPG

JPG - 77 KB

PNG - 77 KB

PNG - 240 KB

Fancy JPEGs

JPEG 2000 - Safari only

Firefox     Chrome     Opera

  • Better quality JPEGs with smaller file size
  • No plans to support by other browsers

A cooler JPEG...
(This image is just a regular jpeg)

JPEG XR - IE/Edge only

Someday maybe:

Screenshots ➔ PNG

PNG - 3.7 KB

JPG - 3.7 KB

JPG - 13 KB

...unless the screenshot is photo-like

Image credit:

Screenshot from Silent Hill 2, published by Konami.

Displayed under fair use.

Animations ➔ GIF

GIF - 4800 KB

JPG - 48 KB

Handy tool:

Someday maybe:

APNG: Animated PNG

Firefox     Safari ✓     Chrome  (new in v59)    Opera (new in v46)

  • The advantages of PNG over GIF, plus animation:
    • smaller file size
    • more colors
    • transparency
  • Shows single frame where unsupported
  • Demo by Mozilla

More than you expected...
(This image is not an APNG)

What about <video> ?

MP4 - 455 KB

<video width="320" height="240" autoplay loop muted>
  <source src="dogeluge.mp4" type="video/mp4">
  <source src="dogeluge.ogg" type="video/ogg">
  Your browser does not support the video tag.

Transparency ➔  PNG

JPG - 26 KB

GIF - 111 KB

PNG - 72 KB

But that's not the whole story...

Icons (and many logos) ➔ SVG

SVG - 0.3 KB

PNG - 4.3 KB

SVG - 0.3 KB

PNG - 2.5 KB

PNG - 2.5 KB

Handy tool:

Bonus: SVG Photo Masks

Handy tool:


Google's Jack of All Trades

Firefox     Safari    Chrome     Edge     Opera

Like an image format clown car!
(This image is not a WebP)

Someday maybe:


Better Portable Graphics

Firefox     Safari ✗    Chrome     Edge     Opera

  • Aims to do everything WebP does, but better
  • Patent-encumbered
  • Info by inventor

This smile's worth paying for, right?
(This image is not a BPG)

(now pay up)

Someday maybe:


New Kid on the Block

Firefox     Safari ✗    Chrome     Edge     Opera

  • Free Lossless Image Format
  • Only 21 months old
  • Claims to be even better at everything
  • No (known) patents
  • Info by inventor

Seventeen months old, and already awesome
(This image is not a FLIF)

Someday maybe:

Optimization Tools

Choose the Right Dimensions

  • Don't make browsers download more pixels than they need - save at proper size to fit layout
  • What about flexible layouts?
    • Not an issue with SVG - infinitely scalable
    • Raster images: save multiple versions

handy tool:

Code Improvements

  • Responsive image dimensions
  • Modern formats with fallbacks
  • Lazy loading with JavaScript

Responsive Image Loading

with srcset and sizes attributes

  sizes="(max-width: 700px) 100vw, 700px"
    monty_100.jpg 100w,
    monty_350.jpg 350w,
    monty_509.jpg 509w,
    monty_638.jpg 638w,
    monty_700.jpg 700w"
  alt="Monty curled up in bed">

What about CSS background images?

Use traditional media queries

.shiba-slide {
    background-image: url("pet-woods_400.jpg");
    background-size: cover;

@media (min-width: 401px) {
  .shiba-slide {
    background-image: url("pet-woods_800.jpg");

Note: This doesn't always work as you might expect

(see Jake Archibald's talk)

possible future option: `image-set` property

Loading Modern Image Types

Use the <picture> element and type attribute

  <source srcset="fancy-doge.webp" type="image/webp">
  <img src="fancy-doge.jpg" alt="Petunia looking very fancy">

Can even combine with sizes and srcset!

    sizes="(min-width: 640px) 60vw, 100vw"
    srcset="fancy-doge_200.webp 200w,
      fancy-doge_400.webp 400w,
      fancy-doge_800.webp 800w,
      fancy-doge_1200.webp 1200w"
    src="fancy-doge_400.jpg" alt="Petunia looking fancy"
    sizes="(min-width: 640px) 60vw, 100vw"
    srcset="fancy-doge_200.jpg 200w,
      fancy-doge_400.jpg 400w,
      fancy-doge_800.jpg 800w,
      fancy-doge_1200.jpg 1200w">

Tips & Notes

Lazy Loading

JS library: lazySizes

  • Pros:
    • simpler development
    • faster initial load (with caveats)
  • Cons:
    • image loading is JS-dependent

Server Settings

  • Gzip compression
  • Cache headers

How can you change these if we don't control server settings?

By choosing your services wisely!

Gzip Compression

Most hosting services now provide this automatically.

Check your site at

(Not recommended with real puppers)

Gzip compresses similar bits...

... into one!

Browser Caching

  • Detailed info from Google
  • Balances load speed (loading from local cache) vs currency (won’t see new versions of cached items)
    • ​Some compromise by choosing a short-ish (a few hours) max-age
    • Some set long max-age and change filenames
    • Some (like Netlify) set 0 max-age, and require etag validation (explanation here)

How's Your Cache?

Workflow Improvements

  • Automation
  • Asset management


  • Generating and optimizing multiple image sizes is time-consuming and error-prone
  • Lots of tool options, but not very organized
    (Spreadsheet compilation here)
  • When comparing tools, consider:
    • Does it require server-side code? Can it be run at build time, or on the fly?
    • Is it hosted by a service? Are images built dynamically? Is there a fee?
    • Does image loading require JS? (Look for
       `data-src` attribute.)

Asset Management

  • Storing multiple image sizes in the repo make it slow to clone, and slow to upload
  • Generating image variants at build time makes for slow builds
  • Git doesn't work well with large binaries: huge diffs
  • Can use standard asset stores (Amazon S3, etc.), but setup can be difficult, and workflow is pretty manual

so overwhelm

Looking to the Future



The Smart Shiba's Guide to Images on the Web

By Jessica Parsons

The Smart Shiba's Guide to Images on the Web

Images take up roughly 60% of average page weight, making them a huge target for performance improvements. I’ll go over several aspects of improving image loading, from choosing the right file type (including some new type on the horizon), to writing markup for responsive image loading, to making sure your images are cached properly. All concepts will be demonstrated with images of my pet Shibas, Monty and Petunia. Presented at SacJS on July 25, 2017

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