Image Optimization: A Comprehensive Roundup - pt.1

In the beginning, the World Wide Web was all about optimization. Standard speed was around 3 kb per second, and hosting space larger than 5 MB was expensive. Then broadband became available for everyone, and web designers grew less and less obsessed with image optimization. Nowadays, younger web developers have almost forgotten optimization principles, so… why not having a quick recap?

Which format is better?

The first images on the world wide web were only in JPEG and GIF format, PNG support was added later by Mozilla (Internet Explorer supported it since version 4, but transparency did not come until version 7). The SVG format added, finally, support for vector images.

We assume that everybody knows about these formats and when to use them, so we’d like to elaborate a bit more on situations in which a particular format is not the best option.


When to use it - Suitable for every photographic image.

When not to use it

  • Small logos or images with few colors are better rendered by PNG or GIF.
  • An image entirely consisting of text should be saved in PNG or GIF format since they produce much crisper edges; a text saved in JPEG looks, in comparison, much more blurry.
  • In general, everything with a lot of sharp and crisp edges is more suitable for other formats than JPEG.


When to use it

  • Primarily with animations.
  • Small icons or small logos with only a few colors.
  • Vector images with sharp edges and few colors.
  • For displaying transparent images with IE6 (PNG transparency is not supported).

When not to use it - Basically, GIF's should be rarely used since in most cases 8 bit PNG performs better.


When to use it - When transparency is required and quality matters.

When not to use it

  • The compression algorithm of PNG is more efficient as size increases, so, for small and simple logos, a GIF may be smaller.
  • When designing for retina displays, two or more versions of the same image/logo/icon have to produced. For vector images (logo, icons), an SVG is a much better choice, since it scales without loosing quality and there's no need to produce additional versions.
  • Non-transparent, photographic images may look better when saved in PNG (it's a lossless format) but they are much larger than the JPEG versions. The loss in quality is minimal and is not enough to justify the sensible "weight" difference.


When to use it - When you need to embed vector graphics that should retain high quality in every situation (typically when designing for retina displays).

When not to use it

  • If you need to support outdated browsers such as IE8 and below.
  • If you don't know what you are doing. Using an SVG file is not as straightforward as using an image and you need to read about the format and all of it's possible buggy implementations. Case in point: Internet Explorer 9, 10 & 11 don't scale SVGs properly (lossless scaling was the main reason to implement it) but there is more than one good workaround out there.
  • Normally, SVG files are very small since they contain just vectors, but this is not always true: very complex SVGs can be quite large and not so easy to render, thus slowing down older machines. Complex files can be optimized directly within the editor (for example Adobe Illustrator has many tools to simplify files).

Typical mistakes

It's assumed that every web designer has had at least some training to understand which image format should be used in different cases, but many developers did not get this kind of training although they have to deal with images, too. So it's not a bad idea to give out a few pieces of advice.

Using PNG for maximum quality may be a bad idea

Some clients, with a limited understanding of how web works, ask for very high image quality and suggest to use PNG, since it's a lossless format. This is a very bad choice: saving in JPEG with a comparable quality produces significantly smaller files without a noticeable difference in quality. Always make tests and compare the results if superior image quality is asked for; you may easily save hundreds of kilobytes per image.

Don't be scared by SVG

SVG is a relatively recent addition, and it's also the most complicated graphical format to handle. That explains why not everyone uses it and, when dealing with a company's logo, the preferred choice is PNG. But using an SVG logo is certainly the best choice around for this kind of graphics. Logos usually are vector graphics: PNG converts it to a bitmap format, while SVG keeps it native. The vector version will be smaller and crisper at every resolution. Recently, SVG has added another point to its score: it's the ideal solution for retina displays. You don't need to have two versions of the same image, one normal and one for high-res displays, as with all other formats: one SVG will fit all.

Sometimes you simply don't have to optimize

Optimization is a word with a positive nuance, but when you optimize you lose something, and sometimes the compromise is simply unacceptable. When producing a full-screen background header image, it's best to make it very big so that it can cover most of the resolutions without scaling up too much. The common breakpoint for that is the typical 1920 x 1200 pixels of high-end displays, and most background images fit this resolution. Unfortunately, when dealing with larger screens, these images often fail to properly fit into the available space because the height of the header stays pretty much the same, while the width is considerably larger. In such a case you need a different background image with a proper ratio. It will be much larger than your original one and you will be probably scared by it's size (but if you use proper media queries, it will be rarely downloaded), but it's better than to show a misplaced header to some users (with high-end equipment).

Optimization tools

We have good news: you can make your assets smaller. There are tools around that excel at shrinking existing images with very little quality reduction. They are so efficient that they should be used in every project, even with a low number of graphic assets. JPEG optimizers have been around for a while but, since the format is already compressed, the gain was negligible. Today, compression algorithms are much more effective and they can reach a decent 10%: if you have many large images, it's a fair amount of saving. But the real sweet part comes about PNGs: one of the first and best known PNG optimizers, Tiny PNG, can normally save around 60%, and it's not uncommon to get even to 80%. This service converts 24 bit PNGs to 8 bit ones; there is a loss of quality but it's usually quite small so it would not be noticed. Considering the huge saving, the trade-off is clearly on its side.

As said before, SVG files aren't always small. Since they are vectors, dimensions do not matter, but if the file is made up of lots of paths and layers, it can become heavy and slow to render. In addition, we need to consider that SVGs are basically XML files and, when generated by vector programs like Adobe Illustrator, they are usually bloated with unnecessary code. An optimization tool, like SVGOMG can do wonders but it has to be used with care because it might remove important pieces of information in the optimization process.

If using an SVG optimizer is not desirable (most of them lack a graphical interface and simple instructions), we can always work in the editor to simplify the file as much as possible and make it lighter. In Adobe Illustrator you can simplify paths, merge paths, and replace Photoshop filters with SVG filters, and don't forget to make the artboard fit the artwork bounds. When exporting the file, we can decide to outline text (this will increase compatibility but the text is not editable anymore), or embed any bitmap image (normally it’s not advised to have any) as well as specify decimal precision; a value of 1 will fit most of the needs.

It may seem strange nowadays to struggle for saving 200-300 Kbytes while we serve our websites out of broadbands capable of a speed of 1 Megabyte per second. One could think of the optimization process as a waste of time and resources but there's nothing more wrong. Have you ever heard of Google PageSpeed Tools or PageSpeed Insights? These are tools given to us by Google to help analyze and optimize our website because speed is a major factor for getting a good page rank. Even if this isn't enough to persuade you to invest time in optimization, you should consider how impatient users have become: they leave a website after few seconds spent on loading. There are many bottlenecks in a typical connection, and an unoptimized website will usually take much more time to load as expected. So with all this mind, are you still sure you want to keep those three 400 Kbytes PNGs in your slider?

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