I had a similar accident where I was at a bank, I backed out of my spot, was stopped, and another car backed up into my door. The accident investigator (localcop w/o authority) came out and accused me of hurrying out of my spot (Because I was a teenager and the guy who hit me was in his 50's). I was driving a sports car and he was in a econobox. The cop said because they didn't see it they had to write both of us a ticket or neither of us. I didn't want a ticket, so, I said fine to 0 tickets.
The insurance company for the guy who hit me later called and asked for more info. I told them I was completely stopped and saw him backing up and he hit me. There were no scratches (if I were moving, his trailer hitch would have scratched across the door). Fortunately, they didn't make a hassle about it and paid for the repairs... well... they wanted to just replace the door skin and not the whole door. Then when I got it back there was a big scratch in the window and I Was able to get them to replace that as well. (Then about 2mo's later my care was stolen and totaled. And I only had 1payment left on it! )
Anyway, If I had gotten some witnesses names I would have been better off, but, in the end it turned out ok. Always get as much info as you can though, you never know who's going to change their story later on. And the more info you have to back up your side of things the better off you are. I never thought about keeping a disposable camera in the car, but, that's a really good idea if I can remember not to let the film expire.
Hope it all turns out ok. Good Luck!