I want to display to the attribute value of ALT tag

The plugin uses the alt-attribute if the image is not providing a title-attribute or caption tag. Ergo: easiest fix is to make sure your theme is not putting out unintended title attributes.

Otherwise you can simply edit the implementation of the getTitle-function in the lightbox JavaScript source:

function getTitle(jqLink){						
			var title = jqLink.attr("title") || ''; 
			if(!title){
				var jqImg = jqLink.children('img:first-child');
				if (jqImg.attr('title')) {
					title = jqImg.attr('title'); //grab the title from the image if the link lacks one
				} else if(jqImg.attr('alt')){
					title = jqImg.attr('alt'); //if neither link nor image have a title attribute
				}
			}
			return $.trim(title);
		}

The plugin is using the minified version of the JavaScript (jquery.lightbox.min.js). I’d recommend you edit the non-minified file and then copy it all over to the .min.js

Thanks for the reply.
I made the corrections as you taught me and the result is exactly what I wanted.
Thanks for your kindness. And Happy New Year to you.