The header location for Atahualpa is: \wp-content\themes\atahualpa\images\header
Your header images can be anysize you want really. You would delete Atahualpa's default images and replace them with your own. The file names don't matter as Atahualpa will recognize jpg, png, gif file extensions
On my site, my header size is 950x200.
Doug
http://www.crazytrike.com