Use the Add/Manage Logo link in the Admin Tools menu at the top of your Brandboard to add an image to the top of the left Brand & Links column on that board.
You can upload a new image or use an image that you've already used as a logo on another Brandboard. The image will automatically be resized to fit the space.
If the image you add is transparent, you'll be able to choose a background color to show through - white, black, or the same grey as the rest of that column. Click the small colored boxes under the logo to choose the background color.
This can be used to add your company's logo, your client's logo, or any relevant image.