When inserting an image into a post, WordPress uses absolute URLs to the files. This plugin overrides that functionality and forces it to use relative URLs.
For example:
WordPress Default: http://www.example.com/wp-content/uploads/2013/04/example.jpg
Plugin Override: /wp-content/uploads/2013/04/example.jpg
This is particularly useful if you plan on switching domains ever as well as reducing HTTP requests.
This plugin is available for download to be used on your WordPress self-hosted installation.