Add Encryption to Images
Encrypted Files module sounds like exactly what you're looking for. From the project page:


  Say you want users of your site to be able to upload and download documents, but you also want these documents to be protected in case someone breaks into your server. This module allows Drupal to encrypt files that users upload and decrypt files for download, keeping the unencrypted versions of files from ever being stored on disk. It does this by creating a custom file stream wrapper that Drupal can read from and write to and a new download method that sits alongside the regular public and private methods. So you can make Encrypted Files the default download method, or only use it as the download method for specific file-type fields.