Add your branding to

Upload company logo

Go to Settings > Agency details > Agency Logo

Once uploaded, you need to refresh your page, for the change to update: 

Change company background

You can simple choose a new background with one of the defaults - see here 

Or you can define your own background image:

This setting will apply to all staff and admin users. Go to control panel, select Agency settings > Other settings. Under Company wallpaper image URL. Add image URL. 

note: Due to security http:// will not work. Only a URL with https://  

Once added - go to the desktop, and right click the mouse to bring up Change background & Theme

Then select in toolbar - Reset to defaults