Cannot upload png photo files into media library. I get a ‘Could not insert post into the database’

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there’s a bunch of possible reasons for your problem. I would check the following:

  1. Make sure that files you’re uploading are allowed by WordPress https://codex.wordpress.org/Uploading_Files

  2. It’s possible that you’re trying to upload files bigger than allowed by your PHP configuration – check upload_max_filesize and post_max_size settings in php.ini. If you do not have access to php.ini on your server, you might need to ask your hosting provider to increase the upload limit for you.

  3. You might have hit the size limit of your database, you need to check that in your hosting control panel.

Maybe one of these would help.