This is the sort of situation that arises because iTunes doesn't automatically do a backup before installing the new OS. It's really stupid on apple's part, Blackberry does this before installing a new OS and that would have solved your problem before it happened.
Getting the pictures back will not be simple. First, the pictures will be recoverable only if the space where they were in the memory chip has not been overwritten. Second in order to know whether they are recoverable, you have to go through a series of operations and you won't know if there's even a chance of success until the end. There's a thread from 2009-2010 at modmyi describing the process, which involves jailbreaking, dumping the iPhone's disk to a file on your computer, and then searching the dump for any picture files that may have survived the upgrade.
Here is the thread.. I don't envy you. File recovery is a difficult operation under the best of circumstances, and Apple makes things difficult because they don't let the device mount like an external drive. This is why you have to jailbreak to achieve a recovery.
I've lost precious family video, though not in the same way. AD video tape containing video of my daughter's first few months of life disappeared after we moved from Maryland to Illinois, and we have no clue why it wasn't with all the other home video tapes which made it cross country safely. I thought the tape might turn up when we moved again two years ago and were sorting through lots of stuff to minimize what we were moving, but it didn't. It sucks beyond words.
Good luck.
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