While I do not have this problem with having my picture taken - I can understand why you dissociate when the flash goes off..... you are reacting as normal as possible to a past traumatizing situation that evolved degrading photos of you.
Keep talking with your T about the up coming event (your wedding) and see what you two come up with.... and maybe someone on here has went through a similar situation and can then give positive feed back for you to use.

LoVe,
Rhapsody - ((( hugs )))

.... Just wanted to let you know that I cared and understand.