WP-LIGHTBOX

Plugin that turns linked images into neat Javascript-powered overlay popups. This is way better than linking to another window, specially for those who have slow connections.
Plugin from M3ntor

For better understanding, I’ve provided some pictures below.

Download Lightbox Plugin. Extract.

Login to your FTP, then Upload lightbox-plugin folder and lightbox-plugin.php in your (wp-content/plugins)

Next, Login to your wordpress account, activate Lightbox 2.02 Plugin

Please read the whole Readme.txt file for more info.