do NOT "SLICE" !!!!!
Hope you are not thinking of "SLICING" -- a bad idea by people who do not want to dirty their hands with HTML, CSS. Slicing a Photoshop or Illustrator or any other type of image creates a filled <TABLE>, and we all know <TABLE> layouts are bad. A filled <TABLE> is even worse.
So-o-o-o-o-o? Cannot figure why the image "mockup" dimensions would matter???