I just hosted a USPSA RO class and this topic came up. According to the NROI staffer who taught the class, Yes, Virginia, you CAN take a sight picture with a loaded firearm.
Make Ready signifies the start of the stage, but the "start signal" is the beep of the timer (in most cases).
To clarify--after "Make Ready", you can't do anything you want. You can't move from the make ready location.