Keith is leaping into the breach with Sandbox. Welcome.
Two tips:
The first is that you can use CSS to do the header image without altering the base theme files.
The second is that when you do edit your custom skin, you should copy just the header/comment section, then @import the base layout, i.e. @import url(’3c-b.css’); in your skin. Makes it much easier to separate your color/style/font whatever hacks from the base structure, and you don’t have to worry about copy and paste problems later if you want to change structure.
The third, unrelated tip, is to install the wordpress atom plugin. (If you do this, you _should_ edit the base theme files to change the discovery links to be accurate.)