In order to style the payment page towards your own brand, it is possible to upload a background image for your Pay by Link page. You can find the upload functionality in the Customer Area > Account > Pay by Link > Appearance > Background image.
Find more information about customizing Pay by Link here: Customize Pay by Link.
Note that in order to see the Pay by Link configuration page, you need the Pay by link Settings role. If this role is not available to you, please ask you admin to add it to your user.