Second customize your template manually by open it with PowerPoint software. You can change a lot of things like slide size, color theme and also header and footer with the "Slide Master" view (be careful in some MSOffice version you can have bad conversion of picture you put in the header and/or footer with the "Slide Master" view. A workaround is to insert the pictures into "shapes": a rectangle for example, by filling it with the picture you want).