The nation's largest police department learned the hard way that there are legions online devoted to short-circuiting even the best-intentioned public relations campaign — in this case, the NYPD's Twitter invitation to people to post feel-good photos of themselves posing with New York's Finest. What #myNYPD got instead was a montage of hundreds of news images of baton-wielding cops arresting protesters, pulling suspects by the hair, unleashing pepper spray and taking down a bloodied 84-year-old man for jaywalking. A similar meltdown came last November when investment giant JP Morgan Chase, which had been paying billions in fines stemming from the financial crisis, asked followers on Twitter to post career advice questions. The #myNYPD misfire comes at a time when new Police Commissioner William Bratton is trying to re-brand the department to counter criticism that it has been trampling on people's civil rights. Last week, it disbanded an intelligence unit that spied on Muslim neighborhoods, and it has promised to reforms to the crime-fighting tactic known as stop and frisk.