Hi,
Although this post should probably have been in the Themes and Templates area, I'll try to point you in the right direction.
First of all, outside of the theme modifications I have done for my own blog, I do not admit to being a theme expert.
That being said, and if you want to make your header look the same on all of your pages, you will most likely need to modify the theme header.php and style.css files to achieve what you want.
As a start, you can get rid of the bar altogether by commenting out or deleting the following code in the header.php file:
<div id="headerbar">
</div>
Other than that, other changes would have to be made -- and there are lots of possibilities.
I do not know what your level of familiarity is with XHTML, PHP, and CSS so it would be difficult to be more specific than this.
What kind of image are you thinking about? You need to be careful to insure that the header you choose is also "fluid." There is lots of information about header design in the WordPress Docs.
I downloaded and looked at the simplicity theme. It looks like a nice theme to work with.
Royce