Image Upload Errors in WordPress 5.3
WordPress 5.3 introduced a new way to manage images uploaded by detecting big images and generating a “web-optimized maximum size” of them.
Unfortunately, this feature seems to have a bug. When uploading ANY image to WordPress, users are reporting the following error: Post-processing of the image failed. If this is a photo or a large image, please scale it down to 2500 pixels and upload it again.
Until a WordPress update is released with a fix for the error, there are two ways to disable it:
Add a function to your child theme functions.php:
add_filter( 'big_image_size_threshold', '__return_false' );
Use a free plugin: https://wordpress.org/plugins/disable-big-image-threshold/
A notice will appear in your WordPress dashboard when an update is available.