it's called image slicing.
If you have any image editing software already installed on your computer (Paint Shop Pro, Adobe Photoshop or Photoshop Elements, Ulead PhotoImpact, Macromedia Fireworks etc) then they can all do this for you.
If you have no software avilable then you can use several free or shareware alternatives. One very good little free program is available from CoffeeCup click here
It allows you to do lots of tricks and one of them is image slicing. It produces the necessary HTML and image files to reassemble your image in a web page, which you can copy and paste into your site.