Hi,
This plugin doesn’t affect the actual image files themselves (unless you use the advanced image compression option). Instead, it changes the markup used to display those images in your posts and theme templates.
Any images you embed in posts you write after installing the plugin will use the responsive image markup, regardless of when that image was uploaded to your site. We wanted to be careful about editing content people have already entered into their site, so we don’t rewrite the markup on old posts. That said, featured images and image galleries on any of your posts should have the responsive image markup applied.
Hope this helps,
Joe
