Well gentel men,i think i have figured it out myself,But i would like 2 share it with u all.
Well here how u can do it
-Make a new compostion
-Bring in the footage U want To apply the picture flash effect
_Go to Layer>new>solid
-From the tools menu(press tab if it is not visible)Take the eliptical mask tool,If rectangle is visible alone,keep the left mouse button pressed and then select eliptical mask tool,Make ur newly created solid layer into an oval shape using that tool.
-from the effects menu give this solid laye a brightness and contrast effect as well as a fast blur effect.Set key frames for fast blur effect.Set key frames for the fast blur effect depending upon ur need.
And here u have a camera flash effect ready,but do add more layers,At random points deepending upon ur need.
U can find a decent camera click sound at the following website:
http://www.partnersinrhyme.com/pir/PIRsfx.shtml
ENJOY