Since I know no way of triggering a script at the end of a drag operation I created a forever loop for each piece which checks whether it is close to its correct position. If so then it jumps to its exact correct position then stamps its image onto the stage then hides itself. This is helpful because the stamped image cannot be accidentally moved and because the stamped image will never hide the timer monitor. A 'score' variable is incremented every time a piece snaps into position and when the score reaches 28 triumphant music plays...

Note that in the program below the timer monitor freezes when the puzzle is completed (instead of displaying the included timer monitor I display the value of a variable called 'time' which is continually updated to contain the value of 'timer' until the puzzle is finished).

I can solve the puzzle in less than 200 seconds - can you?