[jmhart]

For most LCDs, leaving the display off or displaying a full screen display of black (0,0,0) for the same duration as the image that is now persistent will clear it up. If that doesn't remedy it, you can try alternating between 100% white and black every few minutes until the persistent image is cleared.

I've had several LCD projectors that had issues with this "pixel memory" or persistence issue and was always able to remedy it using this simple fix.