Make inserted images in CKEditor automatically responsive

ckeditor-responsive-images.js

// If you want inserted images in a CKEditor to be responsive
// you can use the following code. It creates a htmlfilter for the
// image tag that replaces inline "width" and "style" definitions with
// their corresponding attributes and add's (in this example) the
// Bootstrap "img-responsive" class.
CKEDITOR.on('instanceReady', function (ev) {
    ev.editor.dataProcessor.htmlFilter.addRules( {
        elements : {
            img: function( el ) {
                // Add bootstrap "img-responsive" class to each inserted image
                el.addClass('img-responsive');
        
                // Remove inline "height" and "width" styles and
                // replace them with their attribute counterparts.
                // This ensures that the 'img-responsive' class works
                var style = el.attributes.style;
        
                if (style) {
                    // Get the width from the style.
                    var match = /(?:^|\s)width\s*:\s*(\d+)px/i.exec(style),
                        width = match && match[1];
        
                    // Get the height from the style.
                    match = /(?:^|\s)height\s*:\s*(\d+)px/i.exec(style);
                    var height = match && match[1];
        
                    // Replace the width
                    if (width) {
                        el.attributes.style = el.attributes.style.replace(/(?:^|\s)width\s*:\s*(\d+)px;?/i, '');
                        el.attributes.width = width;
                    }
        
                    // Replace the height
                    if (height) {
                        el.attributes.style = el.attributes.style.replace(/(?:^|\s)height\s*:\s*(\d+)px;?/i, '');
                        el.attributes.height = height;
                    }
                }
        
                // Remove the style tag if it is empty
                if (!el.attributes.style)
                    delete el.attributes.style;
            }
        }
    });
});