The Adaptive Images plugin seems to do it for mobile. Changing the resolutions setting to: 3840, 2560, 1920, 1280, 1024, 640, 480 makes it work for desktop as well.

Since adding https on the blog, my PC struggled to browse the galleries due to having to decrypt all those enormous images; after installing the plugin, I can even browse it on my phone without problems.

The CPU of that instance is hard at work resizing images as you can see and the cache directory is rapidly filling up with resized images, yay!

This is exactly what Cloudinary does. Try their wordpress plugin. The free tier is sufficient for most of the small to medium scale apps.

This looks like a nice option wordpress.org/plugins/fly-dynamic-image-resizer

Here's a part from the plugin description:

How this plugin works

• You either define an image size in your code using the fly_add_image_size() function, or directly call the image size in the code
• The admin uploads the image in the media library, but the fly dynamic images are not created
• The user visits the page for the first time, and the image is dynamically created and is stored
• The user visits the page again for the second time, and the stored version of the image is served

github: github.com/junaidbhura/fly-dynamic-image-resizer

Hope it helps :)

Guys I read all your answers but I think there is one plugin that you've missed; and I am talking about the Jetpack.

This is an amazing well made plugin made from Automattic and from the others it gives you a CDN just for images and it will resize them as well, I advice you to read their API because it gives you so many ideas ;)

Beside that since the 4.4 version you souldn't worry anymore even of the pixels dimension of your images because from now on WordPress supports the srcset attribute to serve different sizes on different devices.

Hope this gives you a little overview of Jetpack because there are plenty of useful feature inside this single plugin ;)