As of version 7.4, the product supports setting logos from within AdminPanel interface.
Log into AdminPanel and open Common screen, Branding tab. You'll find two items there: URL of your logo image (sign in page): If you supply URL here, the image will override standard logo used on login screen. URL of your logo image (top-left corner): If image URL is supplied here, the image will be added into main webmail interface which can be seen once the user logs in.
Keep in mind that image size supported by product design has its limits: width of the image used on login page shouldn't exceed ~300px, and image in top-left corner of email interface will be cut off if its height is over ~80px. Those numbers slightly vary depending on skin used.
Also note that you can adjust site name shown in window title/bar, that's done in Default domain settings; or, if you utilize multiple domains - in properties of that domain in AdminPanel.