Square shadow in center of picture

I recently purchased a Phillips rear projection tv used. It is a bout 4 years old. I bought it from a photographer that had it in his store window. He had a dvd player hooked up to it and stored a square picture in the center of the screen and had it there for quite some time. Once I aquired the television, I have not been able to remove a shadow from the center of the screen from where he had the picture. Does anyoneone have any ideas on how to remove this shadow. Thanks.