This is a further enhancement to the image2 plugin (enhanced image) by CKSource. It includes support for the responsive images attributes srcset and sizes. Two textfields are added to the dialog window, and it is possible to integrate with your custom fileuploader.

1. Download the package.
2. Extract files and save them in ckeditor/plugins/imageresponsive directory.
3. Enable the plugin: config.extraPlugins = 'imageresponsive';

Dependencies: Requires the following plugins to work: Image2, Widget, Line Utilities, Dialog, Clipboard.

Either simply let users set the sizes and srcset attributes by hand, or integrate it with your own filebrowser/quickupload plugin.

You can integrate this plugin further, when using a custom quickupload / filebrowser plugin for CKEditor.

You should implement the logic to save uploaded images in different sizes in your connector. These can either be static sizes or just percentages based on the width of the uploaded image. When using static sizes, you should check for the original dimensions and only use sizes lower than that.

When putting the callback to CKEditor into your custom filebrowser, you can pass in a complete srcset. This is done via the third parameter, which is executed in the scope of the dialog, if it is a function.

window.parent.CKEDITOR.tools.callFunction(callback, "/images/upload.jpg", function() {
    // Get the reference to a dialog window.
    var element,
        dialog = this.getDialog();
    // Check if this is the Image dialog window.
    if ( dialog.getName() == 'imageresponsive' ) {
        // Get the reference to a text field that holds the "srcset" attribute.
        element = dialog.getContentElement( 'info', 'srcset' );
        // Assign the new value.
        if ( element )
            element.setValue( 'upload-small.jpg 100w, upload-medium.jpg 500w, upload-big.jpg 1000w' );
    }
});