Vincent Alvarez, a 32-year-old New York City detective was arrested yesterday and charged with illegally possessing child pornography.  He ordered the pornography from a bogus company that was part of a police and Postal Service sting last May when an investigator mailed him a list of videotapes and an order form, and the detective ordered three child-pornography tapes from the list.  It is not known why the detective was singled out and the criminal complaint does not give the reason,  Alvarez was arrested yesterday and suspended from the police ford, which he joined 12 years ago.  He is awaiting arraignment on charges of possessing a sexual performance by a child and possessing an obscene sexual performance by a child.  Each charge is a felony punishable by a maximum sentence of one and a third to four years in prison.  According to his lawyer, Alvarez will plead not guilty.