If you leave your monitor on with a constant image with out having a screen saver or a dim function you will damage the screen.
Burn-in on an LCD screen occurs when the liquid crystals that generate the colors you see on your screen become stuck in one position. This rarely happens suddenly; rather, LCD screen burn is a gradual process -- the result of leaving a single image on your LCD's screen for not just a few minutes, but for several hours or even days.
solution: Apple has posted a helpful tutorial for removing image persistence on Apple LCDs (though the basic principle should work for any LCD).
1) Create an all-white screen in a graphics application such as AppleWorks or Photoshop, and save it as a JPEG file.
2) Use this as the image displayed by the screen saver.
3) Turn the display brightness down (but not off) to preserve backlight bulb life.
They recommend that you display this white image for as long as the persistent image was displayed. This may not fix things entirely, but it should help remove some of that unsightly burn-in.