jQuery: retain a responsive div size for an image container before fadeIn

Question

I have a bit of jQuery that selects a random image from a folder and fades it in within a div on each page load. The problem is the menu div just below the image jumps up into the blank space of the image before the image fades in.

What's the best way to have the image div retain height before the image fades in? CSS to "fill" the div before the image fades in? Or load a blank .jpg before the random image fades in? (All random images are the same size 1000px width x 250px height).

I need to retain the responsiveness of the design, so it's possible the best solution is to preload a blank jpg and then fade in the random image.

I haven't included the image and caption arrays for simplicity, but this function outputs the image with the .randomheader class:

jQuery:

    <script>
    $(document).ready(function(){
     $('.randomheader').randomImage();
    });
    </script>

<script>
(function($){

    $.randomImage = {
        defaults: {

            path: '/fullpathhere/headerimages/',

            myImages: ['image1.jpg' , 'image2.jpg' , 'image3.jpg'],

            myCaptions: ['Caption 1' , 'Caption 2' , 'Caption 3']    
        }           
    }

    $.fn.extend({
        randomImage:function(config) {
            var config = $.extend({}, $.randomImage.defaults, config); 

            return this.each(function() {
                var imageNames = config.myImages;
                var imageCaptions = config.myCaptions;
                var imageNamesSize = imageNames.length;
                var randomNumber = Math.floor(Math.random()*imageNamesSize);
                var displayImage = imageNames[randomNumber];
                var displayCaption = imageCaptions[randomNumber];
                var fullPath = config.path + displayImage;

                // Load the random image
                $(this).attr( { src: fullPath, alt: displayImage }).fadeIn('slow');

                // Load the caption
                $('#caption').html(displayCaption).fadeIn('slow');
            });
        }
    }); 
    </script>

HTML:

    <header id="masthead" class="site-header" role="banner">

    <!-- random image loads in the div below -->
    <img src="" class="randomheader" width="1000" height="250" alt="header image">
    <div id="caption"></div>

   <!-- The nav div below jumps up into the div above -->
   <nav id="site-navigation" class="main-navigation" role="navigation">
       // nav here
   </nav>

CSS:

.randomheader {
    margin-top: 24px;
    margin-top: 1.714285714rem;
}

img.randomheader {
    max-width: 100%;
    height: auto; 
    display: none;
}

Answer 1

@apaul34208 pretty much hit the nail on the head there... I created an example for you based on the same principle so you can see it working.

HTML:

<header id="masthead" class="site-header" role="banner">
  <div class="randomheader">
    <div class="image"><img src="" /></div>
    <span class="caption"></span>
  </div>

  <nav id="site-navigation" class="main-navigation" role="navigation">
    nav here
  </nav>
</header>

CSS:

.randomheader .image {
  position: relative;
  height: 0;

  /* 
  This percentage needs to be calculated
  using the aspect ratio of your image.
  Type your image width and height in this tool
  to get the aspect ration of your image:
  http://andrew.hedges.name/experiments/aspect_ratio/

  In this example the Google logo was 55:19 and 
  to get the percentage you divide 19 by 55 = 0.3454
  */
  padding-bottom: 34.54%;
}

.randomheader img {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%; 
}

.randomheader .caption {
  display: block;
  min-height: 1em;
}

.randomheader img,
.randomheader .caption {
  opacity: 0;
}

jQuery:

(function($) {

  $.fn.extend({

    randomImage: function(config) {
      var defaults = {},
          config = $.extend({}, defaults, config); 

      return this.each(function() {
        var randNum = Math.floor(Math.random()*config.myImages.length),
            image = config.myImages[randNum],
            caption = config.myCaptions[randNum],
            path = config.path + image,
            $container = $(this),
            $image = $container.find('img'),
            $caption = $container.find('.caption');

        // preload image before adding 
        // to DOM and fading it in
        $image.attr({
          src: path,
          alt: image
        }).load(function() {
          $caption.html(caption);
          $image.add($caption).animate({ opacity: 1 }, 'slow');
        });
      });
    }

  });

}(jQuery));

// on DOM ready, 
// start 'er up
jQuery(function($) {
  $('.randomheader').randomImage({
    myImages: ['logo4w.png'],
    myCaptions: ['Google logo'],
    path: 'https://www.google.co.uk/images/srpr/'
  });
});

JSFiddle Demo