Ok, this is the post many users had asked for. Thesis, as we know, is one of the most wonderful themes around. But putting your own logo in the header is a bit tricky. You may find some tutorials for it, but they are all messy and would require a lot of programming knowledge. I will tell you how to achieve this easily.
Actually, there are two ways to do this. The first one is simple, but it won’t make the logo clickable. The idea is to put the logo as a background to the blog name value, so it can’t be linked to anywhere. The second one is less simpler (remember, I didn’t say difficult 
) ,which overcomes this problem and I personally recommend it. Read on for both options.
Method #1 -
Go to your blog’s /wp-content/themes/thesis/lib/css/header.php file and locate #header #logo .
When you find it, just add these three lines into the class -
background-image:url('path_to_your_logo');
background-repeat:no-repeat;
height: height_of_your_logo
That’s it! Your logo should show up by now!
Method #2 -
Open /wp-content/themes/thesis/lib/functions/document.php and search for <p id="logo"><a href="<?php bloginfo('url'); ?>"></a></p> and give your image url in place of the blog name. So it looks like this-
<h1 id="logo"><a href="<?php bloginfo('url'); ?>"><img src="path_to_your_logo" /></a></h1>
Do the same for <p id="logo"></p>so it looks like this -
<p id="logo"><a href="<?php bloginfo('url'); ?>"><img src="path_to_your_logo" /></a></p>
That’s it! You’re done! You’ve just learnt the easiest way to put your logo in thesis theme. Let me know if it didn’t work, I’d love to see your thesis blogs. Send your Thesis related queries here, we will try to solve them.
Update: New method to add clickable logo in Thesis theme header.
Put following code in your custom.css file.
.custom #logo {background-image: url('images/logo.jpg'); background-repeat: no-repeat; width: 331px; height: 71px; text-indent: -9999px; }
.custom #logo a {width: 331px; height: 71px; display: block; outline: none;}
Note – Change height & width values according to your logo size.