Since I've switched to HTTPS, I've been using Amazon Cloudfront to host my images as it's easy to change from http to https...although there isn't almost a significant improvement in performance, but it's easy for me to change http to https automatically rather than going page by page and changing those embedded images one at a time.
This plugin solved that issue and mixed content errors are gone now.
I would like to thank the creator of this plugin, and although it hasn't been updated since 2013, it does still works amazingly. Now that I fixed my mixed contents error, I've removed the plugin. However, it does not mean it's not working. It WORKS folks, but I have to let it go since I don't need it anymore.
Again, THANK YOU to the developer for having such a useful plugin. It may not have been updated because there is no need for an update. As the usual saying says... If it ain't broken, don't fix it.