You can upload your business logo and create your own custom letterhead for outgoing documents on the web app by navigating to settings>other settings>letterhead. Here you can upload your logo, position it on the page and format how your letterhead looks. Be sure to click save at the top of the page when you are done. You can preview how your letterhead looks using the 'preview layout' button. Once your logo is uploaded you can change it as many times as you like by clicking on 'upload logo' to replace it with a new file. Please note that the logo has a size limit of 200K and must be a GIF or JPEG  file format.

More information can be found by clicking on 'help' on the letterhead page.