<img width="154" height="79" alt="Power CMS" src="../images/logo.png"/>
If you replaced this image with your own and adjusted this tag on the index page you can use your own image.
If you would like to make use of your own css just add it in the list of the loaded css files, so long as it appears last in this list it will take precedence over the default css files.
One can also replace the logo.png with the one in the image folder. I simply make my logo in Photoshop, then save it in the image folder as logo.png. When I do that it asks if I want to replace. I say yes. Then I upload the logo.png to my CMS folder on the server. Keeps it really simple for me.