There are a few plugins that allow you to serve images from your blog posts and pages, however, they only work from when you installed and activated the plugin. What do you do for existing media on your site? This is a great post that provides you with a script, a detailed blog post, and a video to follow along with. By the end of this, your site will be updated, serving your media from Amazon S3!
WordPress and Amazon S3 Quick Fix For Existing Media to be Served on Amazon S3
If you need assistance, please contact us. We can help you with this, as well as any other website needs you have.