danbarber.me.hugo/content/blog/2016-08-10-the-complete-guide-to-optimising-web-images/index.md

---
title: The Complete Guide to Optimising Web Images
date: '2016-08-10T13:23:41+00:00'
categories:
- blog
- images
- web design
layout: post
---

Optimising images for your website can be a bit of a black art, but it's well
worth doing as the bandwidth savings can be large. It will also make a big
difference to the speed at which your site loads.

### Why?

These days many of us have access to very fast broadband (we typically have
100Mb/s connections here at thoughtbot) but not everybody has access to this
sort of connection. Most people worldwide are still stuck on relatively slow
<abbr title="Asymetric Digital Subscriber Line">ADSL</abbr> packages that run at
no more than 2Mb/s or so.

Mobile connections have come a long way in recent years, but the variability of
speed is even worse than wired connections. 4G can be as fast as good broadband,
but in areas with high contention or in areas where there is no 3G or 4G mobile
connections can be as slow as mud.

Whatever the speed of the connection it's worth optimising your images as
[visitors are far more likely to abandon your site if it takes more than a few
seconds to load](https://www.nngroup.com/articles/website-response-times/) and
search engines will rank your site higher if it loads quickly. It's also worth
remembering that even fast broadband connections often have data caps.

<!--more-->

### Size

The first thing to do is make sure the pixel dimensions of the image are
appropriate. Browsers are much better than they used to be at resizing images as
they are rendered, but it is a waste of bandwidth to be sending an image that is
bigger than the size that is specified by the markup or <abbr title="Cascading
Style Sheets">CSS</abbr>.

The context in which the image is displayed should inform the size as well. For
example, a full width background image without much detail may work just as well
if sized a bit smaller and this is an opportunity to save some bandwidth.
Conversely a full-size product shot should probably be sized such that it
retains plenty of detail.

Sometimes it is appropriate to send an image that is bigger than the size it
will be rendered. For example, you might have a 2x version of a logo that is
intended for HiDPI/Retina displays, and the file size may not be significantly
large that it's worth rendering a 1x version as well and adding the complexity
needed to deal with swapping the images.

### Format

So you've finished working on your image, and made sure it's the right size.
What format do you choose to export it? Well, that depends on the kind of image.

When it comes to web images there are a number of formats to choose from, but
the only two you really need to worry about right now are <abbr title="Joint
Photographic Experts Group">JPEG</abbr> and <abbr title="Portable Network
Graphic">PNG</abbr>.

The JPEG file image format was created by the Joint Photographic Experts Group
and, as you might expect, was optimised for storing photographic images. It uses
efficient lossy compression for storing photographic images, as the artefacts
that result from the compression blend well into photographs with detailed
non-uniform textures. These artefacts makes it a poor choice for storing graphic
images and logos however. Because the compression is lossy it means that you
will lose information in the image which cannot be recovered. This means that
you should keep a master copy of your image and only export to JPEG when you are
ready to publish it to the web.

The PNG format was designed as a replacement for the then patent-encumbered
<abbr title="Graphics Interchange Format">GIF</abbr> format. It is a lossless
format, able to store images in a variety of colour depths, with either 1 bit or
8 bit alpha transparency. This makes it ideal for storing graphic images with
strong lines and flat areas of colour. It is also very good at pristinely
storing photographs, but because it's lossless, the resulting file sizes end up
being unwieldy for web use so it's better to use JPEG for photos.

### Optimising JPEG

When saving your images for the web it is usually best to use the "Web
Export..." feature of your image editor. I'll be using Photoshop in my examples,
but most other similar apps have this feature.

<figure>
  <div style="margin-left: -5.6%; margin-right: -5.7%; margin-bottom: -5%;">
    {{< img src="web-export.png" alt="Photoshop's web export dialog box" >}}
  </div>
  <figcaption>Photoshop's Web Export dialogue box</figcaption>
</figure>

The main control you need to worry about here is the "Quality" slider. This
adjusts the amount of compression applied to the image and will affect the
overall quality. Try playing with this control to get a feel for how it affects
the final image. You will also see how it affects the file size of the image
(shown at the bottom). 65-75% is generally a good starting point, but you may be
able to get away with a lower value for certain kinds of image. Conversely you
may have to set it higher to get a result you are happy with. Use your judgement
here.

### Optimising PNG

There's a bit more to think about when saving a PNG file. PNG files can store
images in a variety of [colour
depths](https://en.wikipedia.org/wiki/Color_depth) and with 1-bit transparency
or an 8-bit [alpha channel](https://en.wikipedia.org/wiki/Alpha_compositing).

The first thing you need to decide when creating your PNG file is the target
colour depth. Complex images could potentially require the full 24-bit, but many
images -- particularly simple images like logos -- will work well as 8-bit
paletted images, and will be a good deal smaller too.

Most image editing applications will only let you export a PNG file with an
8-bit alpha layer if you select 24-bit colour. This is a limitation of the
application and not the format. You can use [ImageAlpha](http://pngmini.com) (OS
X) or [PNGoo](http://pngquant.org/PNGoo.0.1.1.zip) (Windows) to reduce a 32-bit
<abbr title="Red Green Blue Alpha">RGBA</abbr> image down to an 8-bit paletted
image while retaining the alpha transparency. Both of these tools use
[pngquant](http://pngquant.org) to do the conversion. Simple images can often be
reduced to 32 or 16 distinct colours, offering further savings!

<figure>
  <div style="margin-left: -5.4%; margin-right: -5.5%; margin-bottom: -5%;">
    {{< img src="imagealpha.png" alt="ImageAlpha on OS X" >}}
  </div>
  <figcaption>ImageAlpha on OS X</figcaption>
</figure>

There are some useful tools to enable you to further compress PNG images:

**ImageOptim**: An OS X application that will run your images through various
lossless compression tools. OptiPNG, PNGCrush, and Gifsicle are some of the
tools that it will use to strip and crush your images down in size without
losing quality. The savings depend on the image you are compressing but can be
anything from 10% to 75% of the original size. ImageOptim will also work with
JPEG images but the savings are usually not that great.

There is a [command line version of
ImageOptim](https://github.com/JamieMason/ImageOptim-CLI) available which would
be useful to incorporate these optimisations as part of a build process.

If you are running Windows you can try
[FileOptimizer](http://nikkhokkho.sourceforge.net/static.php?page=FileOptimizer)
as an alternative. On Linux you can use [Trimage](https://trimage.org). All of
these GUI tools use a similar combination of command line tools to optimise
images.

Happy optimising!

*This post was also published to the [thoughtbot
blog](https://robots.thoughtbot.com/the-complete-guide-to-optimising-web-images).*