Run this JS on selected blog posts types (Tumblr)

Question

JS Total Noob here ! I have a script that adds an overlay on an image by clicking on the image itself. Now I would like to have this script run on all photo posts on a Tumblr.

I want it to run on the Indexpage, where there might be several photo posts in a row. As it is, there is some conflicts and I cannot seem to be running multiple instances on the same page.

I tried to clean my code so that I can illustrate on this JSFiddle. (note that the script is added below the html because it would not work otherwise). So here is a HTML example of a photo post, a text post and a second photo post

http://jsfiddle.net/q3bnxd68

When clicking on the first image it works fine. But the second image created the conflict - clicking on it make an action on the first image.

Any ideas ?

Thank you very much

Here is the CSS

body {background:#000;font-weight:300;}
#wrap{width:90%;margin:50px auto;}


/* ========= Header structure ========= */

#header{width:100%;margin:0px auto 100px;border:0px solid #444;}
#header H1 {margin:0;padding:0;font-weight:300;font-family: 'Open Sans', sans-serif;text-align: center;font-size:2.5em;color:#fff;}
#header H1 a{text-decoration:none;color:#fff;}
#header H2{font-weight:300;font-family: 'Open Sans', sans-serif;text-align: center;color:#777;margin:0;padding:0;font-size:1em;font-style:oblique;}

/* ========= Post structure ========= */

#post-indexpage{}
#post-permalinkpage{}
.a-post{font-weight:300;font-family: 'Open Sans', sans-serif;}


/* ========= Text post ========= */

.text-post{background:#F9F9F3;width:100%;margin:0px auto 0px;
z-index:3;position:relative;
padding-top:70px;padding-bottom:90px;}

.text-title H2 {text-align:center;font-family:'Lato', sans-serif;
font-weight: 600;margin-top:50px; margin-bottom:5px; padding:0px;
color: #42484b;
line-height: 1.3; font-weight:300;
text-transform: capitalize;}

.text-body{border:0px solid #000;width:70%;  color: #222;padding:0px 0px 50px 0px;
font-family: "PT Serif", Georgia, Times, "Times New Roman", serif;margin:0px auto;
line-height: 1.65;text-shadow: 2px 2px 0px #fff;
text-rendering: optimizeLegibility;}

.text-date{text-align:center;color:#999;font-style:oblique;font-size:0.85em;width:100%;}

/* ========= Photo post ========= */

.photo-post{z-index:3;margin-top:-40px;z-index:3;position:relative;}
.photo-img img{width:100%;height:auto;}
.photo-img {width:100%;display:inline-block;}
.photo-caption {display:none;overflow:scroll;font-family:"Open Sans";}
.photo-caption-show {width: 100%;height: 100%;background: rgba(0,0,0,.5);position: absolute;top: 0;left: 0;display: inline-block;-webkit-box-sizing: border-box;-moz-box-sizing: border-box;box-sizing: border-box;padding: 15% 10%;background-repeat: no-repeat;background-position: center center;}

.photo-img img {vertical-align:top;}
.photo-caption p{font-family:"Open Sans";color:#eee;}

And the HTML+JS

<div id="wrap">
<div id="header"><i style="color:#fff; float:right;" id="trigger-overlay" class="fa fa-plus-square-o" aria-hidden="true"></i>

<H1><a href="{BlogURL}">The website</a></H1>
<H2>This is the website baseline</H2>

</div>   <!--CLOSES HEADER -->

<!-- {Block:Posts} -->

<!-- ========= Photo post ========= -->
<div class="a-post" >
    <div class="photo-post" onclick="toggleShow()">
        <div class="photo-img"><img src="http://i.imgur.com/F3ZTcM9.jpg" alt="the alternative text" />
            <div class="photo-caption">
                    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</p>
                    <p> Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla </p>   
            </div><!-- / photo-caption -->
        </div><!-- / photo-img -->

    </div><!-- / photo-post -->
</div><!-- /a post -->

<!-- ========= / Photo post ========= -->

<!-- ========= Text post ========= -->
<div class="a-post">
<div class="text-post">
<div class="text-title"><H2><a href="">This the title of a post</a></H2></div>
<div class="text-date"><em>July 29, 2013</em></div>
<div class="text-body">
<p>Duo an novum impetus scribentur, ius velit aliquando ut. Usu congue commodo delenit ne, pri in perfecto tractatos necessitatibus. Facete perpetua neglegentur nec ei, mei ei affert philosophia. Iisque constituam vel ne.</p>
</div>  
</div>
</div><!-- /a post -->
<!-- ========= / Text post ========= -->

<!-- ========= Photo post ========= -->
<div class="a-post" >
    <div class="photo-post" onclick="toggleShow()">
        <div class="photo-img"><img src="http://i.imgur.com/3thF01e.jpg" alt="the alternative text" />
            <div class="photo-caption">

                    <H2> This is the photo title with a H2</H2>
                    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.</p>


            </div><!-- / photo-caption -->
        </div><!-- / photo-img -->

    </div><!-- / photo-post -->
</div><!-- /a post -->
<!-- ========= / Photo post ========= -->

<script>
var toggleShow = function(){
    var element = document.querySelector(".photo-caption");
    element.classList.toggle("photo-caption-show");
}</script>
</div><!--CLOSES WRAP -->