jQuery Plugin for post loading images

postLoadImages

    // jQuery Plugin for post loading images
	
	$.fn.postLoadImages = function(callback) {
		var imgLen = this.length,
			count = 0;
		return this.each(function(count) {
			count++;
			if ($(this).attr('data-src')) {
				var imgTag = this, imgSrc = $(this).attr('data-src');
				i = new Image();
				i.onload = function() {
					$(imgTag).fadeTo(0,0).attr('src',imgSrc).fadeTo(400,1,function() {
						if (imgLen == count) {
							if (typeof callback == 'function') {
								callback.call(this);
							}
						}
					});
				};
				i.src = imgSrc;
			}
		});
	};

What does it do exactly?
Where should it be located in the HTML-Source - in the head? Before any other JS is loaded, including JQuery?