CSS Snippet for blog post at: http://sp-thornet.blogspot.com/2013/08/jquery-image-lightbox-for-sharepoint.html

dialog_001_gist

/* Lightbox Styles */
/* Lightbox for image hyperlinks */
img.ms-rteImage-LightBox { 
    -ms-name: "LightBox"; 
    width: 12px;
    margin:0px !important;
    cursor: pointer; 
}

/* Lightbox for text hyperlinks */
a.ms-rteElement-LightBox { 
  -ms-name: "LightBox"; 
    text-decoration:underline;
    font-weight:bold;
}

dialog_002_gist

<script type="text/javascript" src="http://sitecollection/Style Library/Scripts/jquery.js"></script>

dialog_003_gist

  				<script type="text/javascript">
						// JQuery snippet for popup box on hyperlinks:
						$("a.ms-rteElement-LightBox").click(function(){   
							SP.UI.ModalDialog.showModalDialog({ 
						        url: $(this).attr("href"), 
						        title: $(this).attr("title") 
						    });
					    return false;
						});


						// JQuery snippet for popup box on images:
						$('img.ms-rteImage-LightBox').on('click',function(){         
						    SP.UI.ModalDialog.showModalDialog({ 
						        url: $(this).parent("a").attr("href"), 
						        title: $(this).parent("a").attr("title") 
						    }); 
						      return false;
						});
					</script>

dialog_004_gist

/* CT: this css stylesheet is referenced in v4.master so I have added 
  the below to fix the MS Modal Dialog on the page 21/08/13 */
.ms-dlgContent {
	position:fixed !important;
}