Oh oh oh, I know how to add header images! I think. The styles team (before my time, ha) tried to add it to the customization area a bunch of styles, including the one you are currently using. Go here:

http://www.dreamwidth.org/customize/options?group=style

To the images section. Look at the "Header image" part--I think the options there are *mostly* self explanatory, but let me know if you have any questions about it!