The image is in the same folder structure as the WSGF widescreen patch.
1. Backup your WSGF provided China_MapOfChina03.jpg if you don't maintain the WSGF Archive
2. Copy the appropriate China_MapOfChina03.jpg from this archieve.
3. Paste the China_MapOfChina03.jpg image into
or driveletter:\GOG Games\Emperor - Rise of the Middle Kingdom\Sierra\EmperorRotMK\DATA
or C:\Program Files (x86)\Sierra\EmperorRotMK\DATA
whichever path your Emperor installation resides.
When done playing with the Japan Mod;
restore the backup China_MapOfChina03.jpg that you created or from the WSGF Archive, whichever method used.