How WordPress handles image uploads.

WordPress image upload process

Whenever you upload an image in WordPress the following happens.

• For each registered image size WordPress creates a copy of the image. Per default WordPress has 4 registered images sizes thumbnail, medium, large and medium large.
• If your image is larger than 2560px an additional -scaled version of the image will be created. This is called the WordPress big image threshold.
• The image gets added to the media library.
• An attachment page is created that makes the uploaded image available in the frontend.

Tip: Disable attachment pages

Each uploaded item also generates an attachment page which is accessible in the front end. We don’t find them necessary and you can use a plugin to disable them.
Here are two suggestions:
https://wordpress.org/plugins/smart-attachment-page-remove/
https://wordpress.org/plugins/disable-media-pages/

Big image threshold

When a new image is uploaded, WordPress will detect if it is a "big" image by checking if its height or its width is above a big_image threshold. The default threshold value is 2560px, filterable with the new big_image_size_threshold filter.

If an image height or width is above this threshold, it will be scaled down, with the threshold being used as max-height and max-width value. The scaled-down image will be used as the largest available size.

Photinia can disable the big image threshold for you via the settings "Big image threshold".