Open the header.php file, or view it in your editor under “Appearance” and look for this line:
<div id="branding">
<div id="blog-title"><span><a href="https://themeshaper.com/demo/thematic/" title="The Thematic Theme Framework" rel="home">The Thematic Theme Framework</a></span></div>
<h1 id="blog-description">Just another WordPress weblog</h1>
</div><!-- #branding -->
I’m guessing you want the logo either next to the blog title, or replacing the blog title? You could do something like:
<div id="branding">
<div id="blog-title"><span><a href="https://themeshaper.com/demo/thematic/" title="The Thematic Theme Framework" rel="home"><img src="logo.jpg">The Thematic Theme Framework</a></span></div>