We can use jQuery to wrap the images under the specified class with a div having the class image-container. Here's a jQuery script to achieve this:

<script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>

<script>

jQuery(document).ready(function($) {

    // Find images within the specified class

    $(".single-post img").each(function() {

        // Create a new div with the class 'image-container'

        var imageContainer = $("<div>").addClass("image-container");

        // Wrap the image with the new div

        $(this).wrap(imageContainer);

    });

});

</script>

This script uses jQuery to target all img elements within elements with the class .single-post and wraps each image with a div having the class image-container.

Make sure to include this script on the page where you want this behavior to occur. You can place it in the HTML's <head> section or before the closing </body> tag, depending on your website's structure and requirements.