Only load images after modal is open

Question

I have a large grid of boxes. Each box opens a modal. Each modal has images inside it.

Problem is that the page loads ALL images and it takes forever. How can I get it to load a modal's images only when that particular modal is opened? Perhaps with a spinner gif as well?

I'm using custombox.js plugin.

$(document).ready(function() {
    $('.info').on('click', function(e) {
        Custombox.open({
            target: $(this).data('href'),
            effect: 'push',
            speed: 500,
            overlayColor: '#2C3E50',
            overlayClose: false,
            //overlayEffect: '',
            //overlayOpacity: 1,
        });
        e.preventDefault();
    });
});

EDIT: example modal code

<div id="modal57" class="modal">
    <div id="portfolioItemClose" class="close"><span></span>
    </div>
    <div class="portfolioTitle wow fadeInLeft" data-wow-delay=".5s" data-wow-duration=".3s">ikuw solutions
    </div>
    <div class="portfolioImageBodyContainer">
        <div class="portfolioImage wow rotateIn" data-wow-delay=".3s" data-wow-duration=".3s">
            <div id="gallery" class="">
                <div class="content">
                    <img data-src="../../../../../assets/images/portfolio/brochures-flyers/20150102_ikuw_flyer-tech_oracle-pl-sql-tips-techniques_1.jpg" class="image_1">
                    <img data-src="../../../../../assets/images/portfolio/brochures-flyers/20150102_ikuw_flyer-tech_oracle-pl-sql-tips-techniques_2.jpg" class="image_2" style="display:none;">
                </div>
            </div>
        </div>
        <div class="portfolioBody wow fadeInDown" data-wow-delay=".5s" data-wow-duration=".3s">
            <div class="portfolioClientDescriptionUsage">
                <div class="portfolioBodyClient wow fadeIn" data-wow-delay=".8s">ikuw solutions</div>
                <div class="portfolioBodyDescription wow fadeIn" data-wow-delay=".9s">PL/SQL tips &amp; techniques flyer</div>
                <div class="portfolioBodyUsage wow fadeIn" data-wow-delay="1s">students</div>
            </div>
            <div class="portfolioBodyText wow fadeIn" data-wow-delay="1.1s">[text]</div>
            <div class="portfolioBodyPDF wow fadeIn" data-wow-delay="1.1s"><a href="../../../../../assets/images/portfolio/brochures-flyers/20150102_ikuw_flyer-tech_oracle-pl-sql-tips-techniques.pdf" target="_blank">View full-scale PDF&nbsp;&nbsp;<span class="fa fa-angle-right"></span></a></div>
            <div class="portfolioBodyLine wow zoomIn" data-wow-delay="1.2s" data-wow-duration=".3s"></div>
            <div class="portfolioBodyVersions wow fadeIn" data-wow-delay="1.3s">pages</div>
            <div class="thumbnail">
                <div class="thumb wow bounceIn" data-wow-delay="1.5s"><a href="#" rel="1"><img data-src="../../../../../assets/images/portfolio/brochures-flyers/thumb_20150102_ikuw_flyer-tech_oracle-pl-sql-tips-techniques_1.jpg" id="thumb_1" class="fit"></a></div>
                <div class="thumb wow bounceIn" data-wow-delay="1.6s"><a href="#" rel="2"><img data-src="../../../../../assets/images/portfolio/brochures-flyers/thumb_20150102_ikuw_flyer-tech_oracle-pl-sql-tips-techniques_2.jpg" id="thumb_2" class="fit"></a></div>
            </div>
        </div>
    </div>
</div>

Answer 1

A solution would be to remove all your modal boxes images sources at load and keep their value in an attribute data-src. Then, when you open a modal window, you could set the sources to only the open modal images thanks to the corresponding data-src value. Something like that:

$(document).ready(function () {
    // Considering that your modal divs have a class 'modal'
    $(".modal img").not(":visible").each(function () {
        $(this).data("src", this.src);
        this.src = "";
        // or remove the whole attribute with $(this).removeAttr("src")
    });

    // Set an attribute `data-sourcesAreSet` to each modal div in order to prevent setting the images sources if they are already set
    $(".modal").each(function () {
        $(this).data("sourcesAreSet", false);
    });

    $('.info').on('click', function (e) {

        var correspondingModal = $($(this).data('href'));

        // Only set the images sources if they are not already set.
        if (correspondingModal.data("sourcesAreSet") == false) {
            correspondingModal.find("img").each(function () {
                this.src = $(this).data("src");
                // or add the attribute with $(this).attr("src", $(this).data("src"))
            });
            correspondingModal.data("sourcesAreSet", true);
        }

        Custombox.open({
            target: $(this).data('href'),
            effect: 'push',
            speed: 500,
            overlayColor: '#2C3E50',
            overlayClose: false
        });
        e.preventDefault();
    });
});

The following is an example of modal box that would work with the above code:

<a data-href="#modal" class="info">Click here</a>

<div id="modal" class="modal-demo modal">
    <button type="button" class="close" onclick="Custombox.close();">
        <span>&times;</span><span class="sr-only">Close</span>
    </button>
    <h4 class="title">Modal title</h4>
    <div class="text">
        Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.
    <img src="Images/example.png" />
    </div>
</div>

If you also want to show a spinner gif while your images are loading, you could do something as suggested in this answer for every image, or alternatively, you could have a div as big as the modal box with an spinner gif inside that you could hide in the custombox's "complete" callback function, though I'm not exactly sure that this function is triggered when all the images have finished loading.