Since Wordpress version 5.3 there was introduced a new module to post-process the images. This was causing a specific error in certain circumstances when you attempt to upload a large image file.
The showing error is:
Post-processing of the image failed likely because the server is busy or does not have enough resources. If this is a photo or a large image, please scale it down to 2500 pixels and upload it again.
You can work around this error trying any or many of the following steps:
1- Make sure the file doesn't have special characters nor spaces in the name .
2- Resize the image if possible to smaller than 2500px.
3- Upload the image through the Media --> Add New menu section in Wordpress before insert it in the page/post.
If you have the knowledge, you can also use the following method:
Edit your Wordpress theme (or child theme) functions.php file and add the following line:
add_filter( ‘big_image_size_threshold’, ‘__return_false’ );
Be careful as you can break your entire site.