If you experienced the error message "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", you should watch the video carefully. After the WordPress 5.3 update, WordPress users are experiencing the issue and looking for the possible solution.
The problem happens for browser caching that you can easily fix clearing the browser cache files. You can also upload images from the Media Library option and the problem will be fixed. I experienced the same issue on my website after the WordPress Version update and did research on it.
Finally, I got a solution and I created the video to help more people who is experiencing the same issue on their website. I know how you feel when you can't upload images on your website and you don't get any solution.