If you are referring to the PowerCMS logo, you can change that in the template.
Under the Template Folder, you will see an admin.dwt.php page, and look for this line of code:
<img src="../images/logo.png" width="154" height="79" alt="Power CMS" />
And then you can either replace the logo.png under the images folder with your image or change the src to point to your image.