Slice it in 3 parts with the center image usually being the repeating image so it expands/collapses to all resolutions. Normally, the center image would be 1px in width but different designs call for different kind of slices but 1px width for the center (and whatever height) should be safe and work without any issues. :)

After i have sliced it where or how do i place them in the header? which css do i look at? Cheers!