Hi Patrick,
if you are meaning here how to change the backup picture of the Lyrics display, please read page 62 of the SX700/SX900 Reference Manual.
You have to call up this function directly from the Lyrics display. I recommend to copy the picture file to the User drive. (Use the Copy function directly in the File Selection display that is called up from the Lyrics display.)
Note: The picture file must be in bitmap format (.BMP), and it can be a maximum of 800 * 480 px. You can also use background picture files originally made for Genos (as mentioned by ckobu before), because Genos has the same limitation of 800 * 480 px here.
Best regards,
Chris