Rokbox is a popular plugin made by Rockettheme for Joomla. It allows you to display images inside a stylish pop-up. You can see the Joomla version of this tutorial by clicking here.
However, there is also a WordPress version available and here's how to use it:
Download Rokbox and Upload to Your Site
Click here to download the Rokbox plugin for WordPress. You'll then need to extract the folder onto your desktop. Upload it to the /wp-content/plugins/ folder on your site
In your WordPress site admin area, go to the Plugins and click "Activate" next to RokBox Gallery
Add an Image to a Post
Go to one of your WordPress posts and click the "Add an image" button. Insert an image into your post. It end result should look like it does below:
View the HTML
Click the "HTML" tab in the top-right of your post.