In this Thesis tutorial there are only few lines of CSS I entered before the lesson and they are 100% for effect. Here is what I have in my stylesheet prior to the lesson:

body.custom { padding-top: 3em; }
.custom #header #logo { text-indent: -9999px; }
.custom #header #tagline { text-indent: -9999px; }

This is from the lesson and goes in the custom_functions.php:

function name_your_function() {
?>
<img id="dog" src="wp-content/themes/thesis_151/custom/images/image.type" alt=""/>
<?php
}
add_action('thesis_hook_before_html','name_your_function');

Be sure to come up with your own appropriate image id. Remember not to use spaces.

Finally, this is the single line of CSS used to position the image:

.custom #dog { position: absolute; left: 33em; top: 1.5em }

Remember to Get Thesis and join the design revolution! Viva La Thesis!