View Source ExTDLib.Object.MessageChatSetBackground (ExTDLib v0.0.4)

A new background was set in the chat.

Name	Type	Description
old_background_message_id	string	Identifier of the message with a previously set same background; 0 if none. Can be an identifier of a deleted message.
background	chatBackground	The new background.
only_for_self	bool	True, if the background was set only for self.

More details on telegram's documentation.