I find that the best way to add a header image is with CSS image replacement on the anchor in the #blog-title div. That way, the image is "clickable". There's a good example of it here. But here's how I do it (you'll want to be dropping the following code snippets into your Thematic Child Theme style.css):
#blog-title a {
background:url(images/image.jpg) no-repeat top center;
display:block;
text-indent:-9000px;
width: XXpx;
height: XXpx;
}
If you want to just add a logo to the left of the anchor you can add some padding to the left and drop in a background:
#blog-title a {
background:url(images/image.jpg) no-repeat top left;
padding-left: XXpx;
}
If you want to just add an image behind the blog title you can use the background selector on #header or #branding. Make sure to use top center like this:
background:url(images/image.jpg) no-repeat top center;
That'll keep your image in place when the screen is resized.
More info can be found under CSS Background on the w3schools site. That site taught me almost everything I know about CSS. That and hacking around with blog templates!
Play around with CSS and experiment. You'll find that it can be really easy to work with—and fun.