Optimize images in WordPress without using any plugin

As your WordPress website grows in content and the traffic to your site is increasing month by month, you might be wondering about your website speed and user experience. You continue uploading your high-quality pictures to amaze your readers. But sooner or later you find that your website has over 1,000 images which are, according to Page Speed Insights, the biggest bottleneck to your website speed.

In this article, you will learn how to optimize large number of images for free and without using any free or paid plugin which could slow down your WordPress site. This article is for absolute beginners and includes code that even non-programmers can run.

Typical solution for image optimization

When you realize that your images are causing issues to your website speed, you start immediately googling and you will find several plugins for image optimization. You can install Smush or EWWW Image Optimizer or even TinyPNG. These plugins usually optimize your images when uploading. But if you want to optimize all your current images, you have to pay a fee. Paying for the plugin which does the job may sound like a perfect solution but, of course, there can be some issues.

Every website hosting, where your WordPress is running, has some limitations. One of the limitations is called Maximum script run time or also in the PHP  max_execution_time. It means how long you can run one PHP script on the server before the script gets timed out by the server and fails.

When you run bulk image optimization to optimize all images inside your WordPress, plugins will optimize images until they hit the limit of the maximum allowed script runtime. If you have a small WordPress website with hundreds of images, there will probably not be any issue. But if you are running WooCommerce website with 1,000 or 10,000 product images, the script will run out of time after one or two minutes and will not be able to finish the bulk image optimization for all your images. In this case, you have to use a different approach.

Solution: Optimize images outside of the server

One of the possible solutions is to download all your images from the WordPress (using FTP) to your computer and optimize your images there. Luckily, there are several tools we can use today. And they are free!

Optimization with Node.js and Imagemin

We are going to use imagemin - a NPM package which allows minifying images and Node.js to run our NPM minifying package over thousands of images.

If you don't have Node.js installed, download it first and install. It's pretty simple. To make sure your node is installed and running, type  node -v into a command prompt. You should get the current version of your node.  

Once you have Node.js installed, create a new folder and give it a proper name. Open your folder in the command prompt and type npm init. The file package.json will be created. This is the initialization file for our project.

Firstly, we have to install imagemin package by running a simple command:

npm install imagemin

Secondly, we have to also install one of the image minifying plugins. I recommend using MozJPEG because it compresses JPEG images in a similar way as the popular WebP format.

npm install imagemin-mozjpeg

Now create index.js file and two folders src and build. You should get this project structure.

Example of the project structure
Project structure

Paste this code to the index.js

// index.js
import imagemin from 'imagemin';
import imageminMozjpeg from 'imagemin-mozjpeg';

(async () => {
	await imagemin(['src/**/**/*.jpg', 'src/**/**/*.JPG', 'src/**/**/*.jpeg', 'src/**/**/*.JPEG'], {
		destination: 'build/',
		plugins: [
			imageminMozjpeg({quality: 55}) // best value is 55
		]
	});

	console.log('Images optimized');
})();

You can change quality but I found out that to get the best visual and size results the 55% is the perfect number.

Before you start optimizing your images, download all your images from WordPress by using FTP. WordPress stores all images in wp-content/uploads.

Once you download images, delete images which are in the format of PNG or WebP because only JPEG images can be optimized with this setup. Put all your images into the src folder and run:

node index.js

Now all your images are optimized and can be found in the folder build. Below is a simple example of the WordPress images optimization. Original image cat.jpeg has been sized down by approximately 80% from the previous original size. The size of other images created by WordPress has been reduced by about 60% to 75% which is a great result.

Images before optimization
Images BEFORE optimization

Images after optimization
Images AFTER optimization

If you're finished with image optimization, upload your images back to the server to the same location wp-content/uploads and make sure to overwrite old unoptimized images. You can now check your website in Page Speed Insights. You should get better score.