Official Responsive Images Plugin for WordPress
Whenever WordPress outputs the image through the media uploader, or whenever a featured image is generated, those sizes will be included in the image tag via the srcset attribute.
Be sure to check out our Screencast about setting up WordPress with responsive images.
Basically, responsive images allow the browser to choose the best image from a list. This plugin works by including all available image sizes for each image upload.
An excerpt from Official Responsive Images Plugin for WordPress
