If I understand you correctly, for posts there is an option for featured image headers (as opposed to featured images for the sticky post slider), but for the pages I need to change things in the CSS. Is that correct?
There is not an option for featured image headers in posts or pages built in to the Fresh & Clean theme. Since there is not a built in option for that, you can use CSS instead.
How do I activate the featured image headers for Fresh & Clean? I've activated a test header, but it will only appear in the sticky post slider (banner = 920 * 116) but the header does not appear in the header at the post. (instead of the title of my site).....
Fresh & Clean does not have featured image headers. It's not part of the theme design.
Explanations and instructions about the features that each theme does have are included in the theme showcase. Here is the Fresh & Clean theme showcase page for reference:
http://theme.wordpress.com/themes/fresh-and-clean/
To change out a header image for a post in the Fresh & Clean theme, first you need to find the post ID for the post in question. You can find it by looking at the body tag in the page source (usually under View → Source in your browser) or by editing the page and looking for the post number in the URL. In the case of http://lotroadventures.wordpress.com/2012/10/02/a-little-test-for-a-new-layout/ the post ID is 923, and the body tag for that post has a class of "postid-923". Using that, you can change out the header image using a rule like this that will only affect the one post:
.postid-923 #branding {
background: url(http://lotroadventures.files.wordpress.com/2012/10/testbanner.png) 0 0 no-repeat;
}
