Make images responsive - create a shortcode

responsive-image-shortcode.html

[responsive]<img src="image_url" alt="alt" title="title" />[/responsive]

responsive-images.php

function responsive_images($atts, $content = null) {
     return '<div class="image-resized">' . $content .'</div>';
}
  
add_shortcode('responsive', 'responsive_images');
Once done, open your style.css file and add those CSS rules:

@media only screen and (max-width:767px) {
    .image-resized img {
        width:100%;
        height:auto;
    }
}