Here, it explains how to specify the background picture of Chatbot window.
Specify background by chat design
(1)Click the setting button on the upper right of the management page.
(2)Click [Chat]>[Design]>[Template] on the left side of the screen.
(3)Click [Display detailed setting]
(4)Click [Choose] button of the content [Background image].
(5)Click [+Add files]
(6)Drop the file that you want to set as background picture of click [Browse] and choose an image.
(7)The screen will be automatically changed, so check to see if the preview screen's background picture is changed as specified.