If you are using SPS 2003 or WSS 2 I would always suggest considering an upgrading to WSS 3 or MOSS 2007, I would especially recommend migrating before implementing graphic changes. But if you really need to implement customisations to SharePoint 2003 sites follow this information from MSDN
Customizing SharePoint 2003 Sites and Portals: Using Templates and Site Definitions
But be warned this is hard, really hard. With SharePoint 2007 you have master pages but wiht SPS 2003 you have to edit 3 xml pages: WEBTEMP.XML,
ONET.XML modifications are are true pain. I really think any IT department should put a freeze on new SPS 2003 customisation.