It is very easy to setup a logo for your invoices, estimates and contracts. Logo is associated to you company profile under the logo section. Within the app go to Settings > General > Profile > Companies , select your company and scroll down to the logo section, then pick your logo image from the Photo library.
Make sure that your logo size is 320×90 (width x heigth) or keep the ratio constant (i.e. 640 x 180). If your logo has different ratio, just adjust it by adding white color where is need it.