95% of the time this error will be caused by uploading huge images or the server not having enough resources to process the image. However it can also happen if settings in CloudFlare are setup in a specific way causing some images to take the full 100 seconds to upload. The error will show but then the image will be in the library after a refresh.
I recommend testing this first by disabling CloudFlare on the site and testing the upload again, if disabling all CloudFlare features fixes the issue then this fix will likely work for you.
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.
- Login to CloudFlare and edit settings for the site in question.
- In the SSL/TLS → Overview tab, Encryption Mode is set to “Flexible”.
- In the SSL/TLS → Edge Certificates tab, the setting “Always Use HTTPS” is set to Off.
- In the SSL/TLS → Edge Certificates tab, the setting “Automatic HTTPS Rewrites” is set to Off.
- Clear all caches.
- Test the upload again.