Those who are interesting in making new banners for either the Classic or Kisasa theme: there are requirements that must be met if we're to fit your banner into place.
For Classic:As KK mentioned, the sides of your banner are going to be faded out. Leave at least 200px (preferably 300px+) of space on either side for the gradients (your banner will preferably end up being around 900px wide, max of 1200px wide). They will be faded to the tan that makes up the rest of the forum header. Your banner cannot be more than 300px high, because anything higher than that just looks silly. Here is the forum banner for the current theme:
http://www.mylionking.com/forum/styles/mlkclassic/imageset/site_logo.jpg.
For Kisasa:The banner you make must be at least 1015px wide and 125px high. (Any image wider than 1015px will have its right side cut off, so you might as well just make it 1015px wide. Maximum height of 300px, because again it would look silly.) The requirements are pretty relaxed, but remember the search bar at the bottom right and try not to have anything in the way of that bottom right hand corner.
If you have any questions please PM me