I left my header's background image URL in there just to show you that if you apply any sort of CSS like that it should work, but you'll want to replace that URL with your own background image's.
To answer your other question, all system styles should accept custom header background images, but the CSS code for adding it in may vary from style to style.
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#header {display:block!important; background:url(http://intoolate.files.wordpress.com/2010/04/header.jpg)!important;}
I left my header's background image URL in there just to show you that if you apply any sort of CSS like that it should work, but you'll want to replace that URL with your own background image's.
To answer your other question, all system styles should accept custom header background images, but the CSS code for adding it in may vary from style to style.
(no subject)