By "found" you mean that they identified a possible suspect as being captured on video, right? Because I don't think they said anything about anyone being in custody, and the latest Times report doesn't mention that: http://www.nytimes.com/2013/04/19/us/boston-set-to-mourn-bom...

So again, why tip off the suspect(s) to anything? I know the pressure for movement on this case is intense, but it seems like the FBI could get away with another day of "We're looking at some leads and talking to witnesses, we hope to make a break etc etc." until the photos were ready to be released without giving the suspects much warning.