Blog.txt add header image

Greetings. Just started with my first blog (…well, not really started!). I’m using the Blog.txt theme, but can’t find a way to change the header into an image. The code, I believe, needs to be changed is:

<div id=”header”>
<h1 id=”blog-title”>/” title=”<?php bloginfo(‘name’) ?>”><?php bloginfo(‘name’) ?></h1>
<?php the_post() ?>
<?php if ( is_day() ) : ?>

<?php else : ?>
<div id=”blog-description”><?php bloginfo(‘description’) ?></div>
<?php endif; ?>
<?php rewind_posts() ?>
</div><!– #header –>

Can anyone tell me where and how to change this code so I can have an image appear as header of my blog?

Thanks!