Actually there are three concepts:
1. Header background - an image (or blank), that cannot be linked to. Think of it like a page background, except that it applies to the header.
2. Header foreground - this could either be a text link (containing the blog title) or an image link to the blog.
3. Description - always a text
See this site: http://www.indiebit.net/
. The "ib" in the circle is actually the logo (foreground), linking to the blog. The rest of the stuff you see in the header is the background (including the text, which is actually a part of the background image). There is no description here.