jquery.randomages.js

(function($){
  
	$.randomages = {
		defaults: {
			path: 'images/',
			images: ['image1.jpg','image2.jpg']
		}			
	}
	
	$.fn.extend({
			randomages:function(config) {
				
				var config = $.extend({}, $.randomages.defaults, config); 
				 return this.each(function() {
					 
						var i_n = config.images;
						var ln = Math.floor(Math.random()*i_n.length);
						var image = i_n[ln];
						var fullPath = config.path + image;
						
						$(this).css({
							'background' : 'url(' + fullPath + ') no-repeat'
						});
				});	
			}
	});
	
})(jQuery);

main.js

$('#zone').randomages({
  path: 'mi_path/images',
	images: ['img_1.jpg', 'img_2.jpg','img_3.jpg']
});