In April 2014 Tim Evko created a WordPress plugin that used the CMS’s capabilities to make using responsive images much easier (just do what you were already doing, reap benefits). He wrote about it here. Later it became the official plugin of the RICG. Now a year and a half later it was merged into WordPress core (meaning when v4.4 drops, it will be in there).
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Responsive Images in WordPress Core is a post from CSS-Tricks