I don't get how you are supposed to kill people with the piano or picture. In fact, where is the picture you are supposed to "accidentally" kill them with?
The picture is in the gardens before you get to the Greenhouse.
It is difficult to describe it in words as you have to see it for yourself. Basically you have to shoot it down and then it falls on top of a guards head LOL. Search for a video on youtube. there is a guy who has done videos for all levels showing you how to complete all challenges. Just search for 'Hitman Absolution Challenges Guide' and go on the video for the first mission.
As soon as you complete a challenge, you can just restart the checkpoint again if you wish. Sometimes this is the better option as some challenges will have everyone around you turning hostile. A good example of this is in the Run For Your Life mission in the library where you have to take the rookie cop hostage. You really don't want all the cops turning hostile in that level, I can tell you!
With regards to the piano, wait until a guard is looking at it and walk towards the piano. You will a message box telling you to press the A button. Once you do this, the piano falls on top of the guard LOL. Very funny death.
Currently playing Hitman Absolution, Black Ops II, Halo 4, Dishonored. Next games I'm buying - Far Cry 3 & Assassin's Creed III