To add a logo or banner to your site, go to "Design->Logos" in the "Design Panel". Select "Use my Portrait" to use the image already uploaded in your account Settings.
To upload a new file, select "Upload my Own" and click on "Browse" to find an image that you’ve saved to your computer. Select the file and click "Open", and then click "Upload" in the "Design Panel". Your image will be added as your site’s logo above your Name.
You can upload .JPG, .PNG, or .GIF images. Your image will be resized if it is larger than 540 pixels wide or 540 pixels tall.