2 things. First, I'm not sure what you're asking for. If you want to use an image for your header instead of text that's not so hard. Problem is I don't understand why you mention the posts table. If you could restate what it is you want to do I'm sure someone around here will help you out.

About your posts table: When you look at your actual database the table named "evo_posts will contain all the posts in the entire blog. One of the fields for that table is "post_category", which is the main category for each post. A post is really assigned to a main category, and each category is assigned to a blog, so you end up with "each post is in a blog" but the truth is slightly different. Also each post can have one or more sub-categories. That information, along with the primary or main category, is stored in the "evo_postcats" table. In that table you will find at line for each post category, meaning 2 lines for post N if post N is in 2 categories. In the "evo_categories" table you will find a field for "cat_blog_ID", which is where b2evo finally gets the post into the correct blog.

Second, I'm going to move this to the "how to" forum because it is more appropriate there.