Good morning Laura,
The entire login page except the upper toolbar can be customized through Domain Settings - Login Page field (see below screenshot). I would recommend you test by creating a JPG image file and uploading it to the domain using the steps of the article I linked above. You would need to create and edit your image file through an image editor program until it looks correct and test it by uploading it to Buzz. Additional adjustments to the image may be needed till it looks correct in Buzz. Alternatively, you could create it through an HTML file and upload it to the domain.