Grumpy_Mike:
Well done.However:-
LDRs are very slow and so there is an added delay built in. Better use a photo transistor or photo diode.
I did this last year with a laser and LDR. I had to introduce a delay so it was timed properly - the speed of the LDR didn't hamper it at all. What did hamper it was having to get the autofocus-only camera to focus, arm the flash, turn the light off, arm the arduino and drop the thing all in a rather cramped bathroom (the only windowless room I have).