If you are an org admin on a business plan, you will be able to add some additional branding to your site and boards.
Via the Brand Assets tab in your Admin Panel, under the Login Theme & Logo subtabs, you can add a login page logo and background, as well as a header logo and footer logo that will show after login at all times, even while viewing a board.
Login Theme
Logo
The best file type for these visuals is JPG or PNG. And, when making any changes here, always remember to scroll to the bottom of the page and click Save.