On the HTTPArchive, Rick Viscomi found that images on sites hosted with WordPress are generally optimized better than on sites that do not use WordPress.
Well, this site uses WordPress. I found a 2.1 MB image of a sea star (2268 × 4032 = 9.1MP) that I took with my phone. I simply uploaded it into WordPress, and placed it into this post:
Downloading this image, it is 655 KB (900 × 1609 = 1.5MP) That is 3.2x smaller in KB, and 6x smaller in terms of pixels! It is also saved at JPEG quality level 96, affording a few more bytes of savings.
Resizing to 400×715: The image is saved at 400×715 – a teeny 204KB.
So, when using WordPress, it appears that the site automatically resizes the images to exactly fit the dimensions appearing on the screen, saving KB of data, and the device from having to resize the image to fit on the screen!
Running this page through WebPageTest, we see that there are dimension parameters added to the requests – which allows the server to resize on the fly:
And don’t let the extension fool you. They were also sent as WebP.
Interestingly, the screenshots from WebPageTest (pngs) remain as PNG files.
So, even without any WordPress Plugins, WordPress is optimizing images to improve speed and loading of their sites.