On your chat screen right click for a dialog to open.
Choose adjust background.
When the dialog opens you will see a select background button when you click that you can then navigate to where you have saved your .bmp image to use as a winbus background.
You can then select the background position on the screen.
You can then close the dialog, and enjoy your new background.
Also you can select Preferences / display and choose a background there.
Enjoy this Premium Feature, and more to come!