From the image you provided i honestly dont see any shadow appearance ^^; not that you arent seeing it, just maybe its your Actual monitor, a light could be out, or a dent in screen i know happens at times to cause a improper shadow effect (Works at a electronic store so i deal with this alot)
Main problem is how its hooked up as well, if it is a desktop and hooked up by VGA ur cord could be simply loose or breaking~ Double check all your cords are properly secure and try again
You can also change the refrsh rate on your computer. Right click on your desktop, click on properties, then choose the settings tab. Now press advanced, then click the monitor tab. Change the refresh rate