In your header.php file there should be a line like this:
<div id="header">
<div id="headerimg">
<h1><a href="<?php echo get_settings('home'); ?>"><?php bloginfo('name'); ?></a></h1>
<div class="description"><?php bloginfo('description'); ?></div>
</div>
Remove the following:
<h1><a href="<?php echo get_settings('home'); ?>"><?php bloginfo('name'); ?></a></h1>
That removes the title of the blog. The other line is for the tagline.
To add a image, not as a background, just add html such as this to your header.php :
<img src="imagename.extention">
To keep your document valid, you will have to set the size & attributes of that header image in your style.css. This can be done by using this code in your header.php :
<img scr="imagename.extention" class="headerimg">
and this code to your style.css :
.headerimg {
width:700px; /* if you want it 700px wide */
height:100px; /* same as above */
margin:0px; /* to make it fit nicely :) */
}
But if your using a image as a background will move your tagline text below the image. So in my opinion it would be better to use the background image on the headerimg id you want to use.
