Custom design of GUI web elements to CSS/HTML/JavaScript? How?!?

I've got loads of beautiful GUI web elements downloaded from the Internet but not sure how to replace and implement them with a current existing one. Well they come in one piece of image, do I need to slice them one by one by Photoshop first and then keep the files where? How to call and use it in CSS/HTML/JavaScript??! Please explain or tell me the source as I'm a programmer but quite new in web developing industry.