Head to WP Accounts settings link and you will be presented with a number of settings, all of which are easy to use and manage.
The first fields are self-explanatory. The only element I would flag up is the logo size. This gets added to the top of your invoices and too big it will swamp the layout. Best thing to do is to add one that looks about right and then do a test invoice to check it looks correct.
Again this is easy to understand. The Paypal link is obtained from the admin users email address.
Image 3 gives you the details you can control when setting up an image template. The settings in image 4 give you control over a couple of useful elements. If you insert a place ID then this will be added to the invoice and allows customers to visit that link and review your business or read reviews etc.