Navigate to ADMIN > SITE > SITE LOGO (under the "More Information" heading)

Note: You can type in the name of a media file you have previously uploaded, or you can upload a new image and it will save the logo in your media files.

It is recommended to use .PNG file version of the logo.  A PNG file will allow for any transparency in your logo to show up.

The logo size is set to a max width of 420 pixles wide by 80 pixles high. If you uploaded a logo image that is larger than the recommended image size, your logo will be resized to a max width of 420 pixels wide.