Hi @vincentchang
If the pictures are OK when downloaded from the SD card and viewed on a computer etc then there's either a problem with the LCD screen or the pcb board that interfaces between the mainboard and the LCD screen.
I can't find the service manual for the camera but given the cost of a [https://www.google.com/search?client=firefox-b-d&q=Sony+Cyber-shot+DSC-W690+LCD+screen|replacement screen] (examples only) it may be worthwhile considering replacing it and check if that resolves the problem.
If the screen is the problem then it will save a lot of time that may be spent looking for the problem elsewhere in the camera, that isn't there.