To change the logo, you must upload your logo, and then change the stylesheet to point to it.
Upload your logo as an attachment to one of your tables. Copy the url of the attachment. It will look something like this:
Go to Design > Skins.
You will see the stylesheet file for the current stylesheet. Search for #logo then change the url of the location that the image is coming from to your image as follows:
12/25/2007 12:45:00 AM