Optimizing images for the web - an in-depth guide

Adrian Bece on October 01, 2019

Table Of Contents Calculating JPG image file size Online image optimization Automated solutions Image loading optimization Using CDN We... [Read Full]
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Thanks for that awesome article giving a nice overview.

Would appreciate a full example combining a picture element for different screensizes, formats (WebP) and pixel densities. As far as i understood this article, that should be the best solution (adding CDN, lazy loading and so on...).


Thank you very much.

I've copied this directly from one of my projects. Hopefully, it will be helpful to you. In the article, I didn't want to overwhelm the users with the full example, but I might include it as a bonus.

    <source srcset="./images/webp/hero-image-420-min.webp 1x, ./images/webp/hero-image-760-min.webp 2x" type="image/webp" media="(max-width: 440px)">
    <source srcset="./images/minified/hero-image-420-min.png 1x, ./images/minified/hero-image-760-min.png 2x" media="(max-width: 440px)">
    <source srcset="./images/webp/hero-image-550-min.webp 1x, ./images/webp/hero-image-960-min.webp 2x" type="image/webp" media="(max-width: 767px)">
    <source srcset="./images/minified/hero-image-550-min.png 1x, ./images/minified/hero-image-960-min.png 2x" media="(max-width: 767px)">
    <source srcset="./images/webp/hero-image-420-min.webp 1x, ./images/webp/hero-image-760-min.webp 2x" type="image/webp" media="(max-width: 1023px)">
    <source srcset="./images/minified/hero-image-420-min.png 1x, ./images/minified/hero-image-760-min.png 2x" media="(max-width: 1023px)">
    <source srcset="./images/webp/hero-image-760-min.webp 1x, ./images/webp/hero-image-960-min.webp 2x" type="image/webp" media="(max-width: 1919px)">
    <source srcset="./images/minified/hero-image-760-min.png 1x, ./images/minified/hero-image-960-min.png 2x" media="(max-width: 1919px)">
    <source srcset="./images/webp/hero-image-960-min.webp" type="image/webp">
    <source srcset="./images/minified/hero-image-960-min.png">
    <img  src="./images/minified/hero-image-960-min.png" alt="Example">

Hey Adrian, have a look at conditional webp returning. You can see browser headers for Accept and if webp appears there when the request for an image is made, you can conditionally return the webp to the client. Browsers will render the webp instead of a jpeg for example. No code changes required, just pipeline changes to create a webp alongside your jpegs, then add the condition to your webserver.


I had no idea it can be implemented server-side as well. Thank you very much for your suggestion.

I found this guide, looks simple and straightforward:



This can cause issues when saving images offline (eg for building offline-capable PWA's) unless you're careful to change the filename extension when you store it offline, eg storing a .jpg file with webp contents can trigger security checks since the mime-type of the contents of the file doesn't match with the mime-type of the file extension (extension sniffing has to be used since there's no mime-type headers for file://)


Hey Adrian in my case i am running an blogger blog where it's full of YouTube videos, what I am doing is using staticaly cdn where I can insert there cdn.statically.io/img/:image_url to optimize images


CDNs that optimize images right out of the box like Cloudinary and Statically are awesome. In cases like yours, CDNs really take a weight off our shoulders.

Are you doing any loading optimization like lazy loading the images?


No because i am using this blogger platform with a amp theme,still google is not officially enabled amp for blogger platform,struggling a bit to implement things for a fully video blog like mine in i think you know if i add a video from YouTube as blog post it will have 2 URLs one is for image and the second one for YouTube video, in my case image URL is used for thumbnails in homepage and when sharing in social media it will be used

I have spoke with the lots of amp theme creators still they don't know how to make valid YouTube amp post.


If you know how to use lazy loading in blogger please tell me how to implement it


Although this post mentions native lazy-loading, seeing as it is already available in Chrome for desktop and Android (that's over 50% browser support! caniuse.com/#feat=loading-lazy-attr) - I think it'd be wise to conditionally load JS library for lazy-loading only when native isn't available, more info on that here: web.dev/native-lazy-loading


Also, to improve image loading - remember to set the width and height attributes for <img>, more info in this video from Jen Simmons of the CSS Working Group: youtube.com/watch?v=4-d_SoCHeWE


Thank you very much for the info on implementing the native lazy loading with fallback.


Interesting list!

Do you know of an image compressor that also converts .jpg into webp? You can also install ImageOptim on your computer and compress images before uploading them.

I'm also curious why you only include the picture element and not img srcset? From my reading of the MDN spec, picture is for the art direction problem, and img srcset is for image resolutions.


Thank you very much.

I've used picture element because it offers me more control over which image is being loaded. I guess I prefer explicitly telling the browser what to load. I've found it to work great with both art direction and performance. Also, picture element allowed me to support WebP with a really nice fallback.

I think it's just a matter of use-cases and personal preference.


What is your thought about adding images via background in CSS?


Can images added via this method be optimized using picture or srcset, or do you just have to optimize them by resizing and compressing? thx

For this method, my primary concern is accessibility. Without the alt tag and any HTML, a simple div with background-image doesn't have good accessibility.

Another issue is that you'd have to use an image aspect ratio to calculate the padding (spacing) needed for the background image which may lead to code bloat if you have lots of images with various aspect ratios.

As for optimization, we need to optimize them by compression and resizing (if needed).


I've been wondering - how much time would love to confess about the fact that lazy loading is cool? Well, because it is very shabby looking, permanent screen blinking, the gray squares, which is 2000 on dialup modem sit and wait for the download. Well, that's pathetic! Did the developers do not have own opinion and remains only an opinion which they impose on internet giants?


The described not optimized scenario it is actually describing a bit map image (bmp) a format that is uncommon and old and it is unlikely to happen.

I didn't know there was native lazy loading, thanks for the tip.


Thank you.

Are you currently working on a project that uses lots of images?


Not a lot but one of my class project use some, I was particularly interested in the lazy loading think especially the progressive loading 👌


This has singlehandedly been the most detailed post I ever read on image optimization. Thank you!

I've been using so many freaking tools and methods, forgetting what I did, and then googling it and doing something else. I'm going to be revisit this post for a long time!


I'm using this service statically.io/imgpx, It has a lot of features I need. And now it supports custom domain.


Someone else also mentioned statically. I might use it on my personal projects if needed. Are there any downsides to using that particular service? Any issues that you ran into during development?


Sometimes there's an error on some files that are cache from Github. So I must rename it on Github and recache it again. I still don't know why.


Thanks for that article. I hope you will post more new idea for old solution !
Thanks you so much !


Thank you very much, hope you'll find it useful in your projects


Hi Adrian, I'd like to translate this excellent guide to Chinese. The translated text will be published at nextfe.com
Can you give me the permission?


Hello Jang. Can you please contact me via email? My email is adrianbece@live.com

I would like to know more about your website and what is your publishing policy. Do you link to the original article and is the author credited?

Thank you


The author is credited at the beginning of the translation, and is linked back to the original article. I've sent you an email with details. :-)


That image at the end reminds me of two awesome albums. FM-84 Atlas and GUNSHIP :D


I love the outrun / synthwave / vaporwave style. That's the reason I've used the image.


Yesss! Me Too! Though I've never known the source of the album art for those two albums, those images are great 👌

I'm also a huge fan of LIGHTS as well. Always on the look out for people who have a shared love of LIGHTS 👍

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