Latest Update: The plugin has been merged into WordPress core. So if you're running WordPress 4.4 or newer, you automatically have this.
Update: The plugin created in this article has moved here and now uses the more appropriate
srcset attribute. It's the official WordPress plugin of the Responsive Images Community Group and is endorsed by the WordPress Core Team. It sounds likely that this will make it into WordPress core eventually. Pretty cool!
The following post is guest co-authored by Tim Evko (@tevko). WordPress has a built-in media uploading system. When you upload an image, it automatically creates and saves different versions of it. Tim will show us how we can customize and exploit that ability to help us with responsive images in content.