I would do this with jQuery (and I bet you're using jQuery for your template too :) )
Be sure you're calling the jQuery library in your page, I would recommend to place it just before the </body> tag and BELOW all the scripts.
for example
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<!-- let's call the following div as the POPUP FRAME -->
<div id="popup">
<!-- and here comes the image -->
<img src="http://i.imgur.com/cVJrCHU.jpg" alt="popup">
<!-- Now this is the button which closes the popup-->
<button id="close">Close button</button>
<!-- and finally we close the POPUP FRAME-->
<!-- everything on it will show up within the popup so you can add more things not just an image -->
</div>
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script>
//your jquery script here
</script>
</body>
</html>
This will show up a piece of code, if you want to simply show an image, put the id="popup" directly on your <img> tag.
Now, let's move to the example... the code is pretty easy to understand:
//with this first line we're saying: "when the page loads (document is ready) run the following script"
$(document).ready(function () {
//select the POPUP FRAME and show it
$("#popup").hide().fadeIn(1000);
//close the POPUP if the button with id="close" is clicked
$("#close").on("click", function (e) {
e.preventDefault();
$("#popup").fadeOut(1000);
});
});
The script behaves like this: When the page is loaded, the content inside <div id="popup"> show up, and if the button with id="close" is clicked, then the pop up is hidden. Add whatever you want inside this <div id="popup"> and it will show inside the popup.
The CSS: SUPER IMPORTANT!
/*we need to style the popup with CSS so it is placed as a common popup does*/
#popup {
display:none;
position:absolute;
margin:0 auto;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
z-index: 9999;
}
You can see it working along with the HTML on this live example:
http://jsfiddle.net/Lp9edyg5/1/
Answer from Frondor on Stack OverflowVideos
Can I customize the popup?
Can I add buttons to the popup?
Does this work in WordPress?
You can use a lightbox to do it.
There is a simple example in this link:
http://lokeshdhakar.com/projects/lightbox2/
You can use a lightbox.
It's really simple to use. Include the script tags and CSS stylesheets in your HTML file, and then simply use:
<a class="example-image-link" href="images/image-1.jpg" data-lightbox="example-1"><img class="example-image" src="images/thumb-1.jpg" alt="Girl looking out people on beach"></a>