The location of the header sponsor image can be seen here on the front end.
STEP 1
Go to Site Options from the dashboard menu
STEP 2
Click on the "Header Sponsor" label
STEP 3
Click "Remove" button, then add a new image from the media library or upload a new image from your hard drive
NOTE: Please use an image/logo that is 100 x 50 px. This will prevent it from being distorted or cropped.