1. Log in to your dashboard--> Design- -> Edit HTML
 
2. Search: ]]></b:skin> and before it paste this code:

/* LightBox */
#jquery-overlay{position:absolute;top:0;left:0;z-index:90;width:100%;height:500px}
#jquery-lightbox{position:absolute;top:0;left:0;width:100%;z-index:100;text-align:center;line-height:0}
#jquery-lightbox a,#jquery-lightbox a:hover{border:none}
#jquery-lightbox a img{border:none}
#lightbox-container-image-box{position:relative;background-color:#fff;width:250px;height:250px;margin:0 auto}
#lightbox-container-image{padding:10px}
#lightbox-loading{position:absolute;top:40%;left:0%;height:25%;width:100%;text-align:center;line-height:0}
#lightbox-nav{position:absolute;top:0;left:0;height:100%;width:100%;z-index:10}
#lightbox-container-image-box &gt; #lightbox-nav{left:0}
#lightbox-nav a{outline:none}
#lightbox-nav-btnPrev,#lightbox-nav-btnNext{width:49%;height:100%;zoom:1;display:block}
#lightbox-nav-btnPrev{left:0;float:left}
#lightbox-nav-btnNext{right:0;float:right}
#lightbox-container-image-data-box{font:10px Verdana,Helvetica,sans-serif;background-color:#fff;margin:0 auto;line-height:1.4em;overflow:auto;width:100%;padding:0 10px 0}
#lightbox-container-image-data{padding:0 10px;color:#666}
#lightbox-container-image-data #lightbox-image-details{width:70%;float:left;text-align:left}
#lightbox-image-details-caption{font-weight:bold}
#lightbox-image-details-currentNumber{display:block;clear:left;padding-bottom:1.0em}
#lightbox-secNav-btnClose{width:66px;float:right;padding-bottom:0.7em}

3. Then find </head> and before it paste :

<script src='http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js' type='text/javascript'/>
<script src='http://artistutorials.googlecode.com/files/lightbox.min.js' type='text/javascript'/>

If you already have a jquery script on your template , you can delete the red line.

Save the template.

DEMO
You can see the working effect on this site, just click on the image.