tinymce.init({
  ...
  image_caption: true, //turn it on
  //image_dimensions: false,
  //image_title: true,
  //images_upload_credentials: true,
  ...
});

// Function (jQuery dependent) to wrap images (subject to selector)
// with <figure> and insert <figcaption> with image's alt text content
function addImageCaptions(selector) {
    var $captionedImages = $(selector);

    $captionedImages.wrap(function() {
        $this = $(this);
        var imgClasses = $this.attr('class');
        $this.removeAttr('class');
        return '<figure class="' + imgClasses + '"></figure>';
    });
    
    $captionedImages.each(function() {
        $this = $(this);
        $this.after('<figcaption>' + $this.attr('alt') + '</figcaption>');
    });
}

// Define the image selectors
var imageCaptionSelector = '.accentImageLeftCaptioned, .accentImageRightCaptioned';

// Add the captions - run this after DOM is ready
addImageCaptions(imageCaptionSelector);