This is a pretty common task. But one that there may be some “mystery” around if you haven’t done it before. It is actually pretty simple once you get under the hood. Check out these directions to find out how to create your own custom theme (based upon an existing one) quickly and easily.
Note: when you are editing the server XML file and putting in the new entry for your images, the images folder referenced is actually up a level from your themes directory. The existing ones start with TH (there is a ton of other stuff in that folder). Grab an existing one to get your dimensions.