Login to your Dashboard and select Settings on the left hand menu
Select Customise from the tab indicated in the screenshot below
Select "browse" to choose a file from your device
BUSINESS LOGO: File type : JPG or PNG, Maximum size: 2MB, Images will be cropped to 200 x 200 pixels.
BACKGROUND IMAGES: File type : JPG or PNG, Maximum size: 5MB, Images will be cropped to 480 x 800 pixels.
Then select "Save" to close"