I can see how adding a personal touch to those templates could help your business exemplify its style, so I'm sending a feature request up to the development team for consideration. If it's a feature that's necessary for your business, I highly encourage them to send feedback directly to our Product Development Team:
Click the Help menu.
Select SendFeedback Online, then Product Suggestion.
Follow the on-screen instructions.
Lastly, I don't want you to go empty-handed. You may want to visit our Apps Center. In here, you can look for a third-party app, that will let you add a picture-type watermark to your invoice and integrate it with QuickBooks.
Keep in touch if there's anything else I can do for you. I always have your back and am always happy to help. Have a great day!