How to manually add to a theme?

Sadly, this doesn’t work out of the box for me. I’m building a site using this theme: http://www.wunder.umbrella.al/light/

There are two places where images occur that I’d love to have fire up in lightbox if a user clicks on them:

1. Posts. Example: http://www.wunder.umbrella.al/light/?p=223
2. Projects. A custom post that includes a slider: http://www.wunder.umbrella.al/light/?work_post=pink-poker

I looked in the theme files for where the single post image and project slider occurs. It’s in single-post.php and single-work_post.php respectively. I even tried to add rel=”lightbox” myself. But I probably got it wrong, because it’s php code and I don’t know exactly how to place the code.

This seems to be the code for the posts image:

								<div class="blog_image">
									<img />" alt="#"/>
								</div>

This seems the code for the project slider:

							<ul id="slides">
                                <?php
                                    $images = get_field("image_slider");
                                    if($images):
                                    foreach($images as $image):
                                    ?>
                                        <li><img />"/></li>
                                    <?php
                                    endforeach;
                                    endif;
                                ?>

Can you give any guidance on how to add rel=”lightbox” to these two snippets of code?

Thanks so much!