Adding an image logo instead of the text site title is simple. Add the following code into the custom.css file which is located in the /wp-content/themes/thesis_16/ directory.
#logo a {
background: url(images/logo.gif) no-repeat center center;
width: 295px;
height: 61px;
text-indent: -9999px;
display: block;
}
#tagline { display: none; }
The last line will hide the tagline. I prefer not using this last link and instead just hiding it from the Thesis Options page in the Header section.
Also, be sure to change the width & height to match your logo.