(function($){  
	$.fn.extend({   
		rollover: function() {   
			return this.each(function(i, el) {  
				if(el.nodeName == "SPAN") {
					if($(el).data("fileName") == undefined) {
						var tmp = el.css("filter").split("'")[1];
						$(el).data("extension", tmp.pop());
						$(el).data("fileName", tmp.join("."));
					}
					$(el).hover(
						function() { $(this).css("filter", "progid:DXImageTransform.Microsoft.AlphaImageLoader"+"(src=\'"+$(el).data("fileName")+"_hover."+$(el).data("extension")+"\', sizingMethod='scale')")	},
						function() { $(this).css("filter", "progid:DXImageTransform.Microsoft.AlphaImageLoader"+"(src=\'"+$(el).data("fileName")+"."+$(el).data("extension")+"\', sizingMethod='scale')") }
					);
				} else {
					if($(el).data("fileName") == undefined) {
						var tmp = el.src.split(".");
						$(el).data("extension", tmp.pop());
						$(el).data("fileName", tmp.join("."));
					}
					$(el).hover(
						function() { $(this).attr("src", $(this).data("fileName")+"_hover."+$(this).data("extension")) },
						function() { $(this).attr("src", $(this).data("fileName")+"."+$(this).data("extension")) }
					);
				} 
			}); 			
		}
	});       
})(jQuery);

(function($){  
	$.fn.extend({   
		rollover: function() {   
			return this.each(function(i, el) {  
				if(el.nodeName == "IMG") {
					if($(el).data("fileName") == undefined) {
						var tmp = el.src.split(".");
						$(el).data("extension", tmp.pop());
						$(el).data("fileName", tmp.join("."));
					}
					$(el).hover(
						function() { $(this).attr("src", $(this).data("fileName")+"_hover."+$(this).data("extension")) },
						function() { $(this).attr("src", $(this).data("fileName")+"."+$(this).data("extension")) }
					);
				} else if(el.nodeName == "SPAN") {
					if($(el).data("fileName") == undefined) {
						var tmp = el.css("filter").split("'")[1];
						$(el).data("extension", tmp.pop());
						$(el).data("fileName", tmp.join("."));
					}
					$(el).hover(
						function() { $(this).css("filter", "progid:DXImageTransform.Microsoft.AlphaImageLoader"+"(src=\'"+$(el).data("fileName")+"_hover."+$(el).data("extension")+"\', sizingMethod='scale')")	},
						function() { $(this).css("filter", "progid:DXImageTransform.Microsoft.AlphaImageLoader"+"(src=\'"+$(el).data("fileName")+"."+$(el).data("extension")+"\', sizingMethod='scale')") }
					);
				}
			}); 			
		}
	});       
})(jQuery);

What it does is look for img tag and on rollover change the source path from "anyimage.jpg" to "anyimage_hover.jpg". It add the suffix "_hover" at the end of the src name without changing the extension. If you look at the code you will see that it also take the span tag. This is to support ie6 png image that has been transform by a pngfix.
Example: $("img.rollover").rollover();

jQuery RollOver plugin

javascript, plugin, jquery

jQuery