To add an image to the top of your invoices navigate to Settings > Brands > Choose the brand > Click the + button to Upload a background Image.
Preferred size: 950x250
Max image Size: 500KB
This image will show on all of your invoices for this brand.
The logo you have here will also be on all of the invoices.
Preferred Logo size: 160x160
Max image size: 500KB
