Find the "More" function button (usually three vertical dots or "…" symbol) in the upper right corner of the chat window and click it.
In the pop-up list of options, select "Chat Settings" or "Set Chat Background" option.
After entering the chat setting interface, you can see the option of "Chat Background Historical Map". Click it.
At this time, a window of chatting background history map will pop up. You can browse and select the background image you want to set.