Here is what might have happened:
You had a drive that was formatted as a PC drive and the Mac OS, thinking it was helpful, offered to back itself up to that drive. If someone clicked yes several times, that would prompt you to erase the drive (and since you didn't list the OS running on the Mac, the exact terms might be different if you have Lion versus Mountain Lion vs another OS version).
At that point, the drive would get reformatted, but the vast majority (perhaps all) of the files on the drive would be still intact, but no longer linked to a directory.
If the drive was ejected before it could write the first backup, you would have great luck using a commercial file recovery program. On Mac, you would get Data Rescue 3 or similar to undelete the files.
If the Mac couldn't erase the drive, that might be the best case for you since it wouldn't start backing up (which will begin to write new files on all the "free space" that it thinks contains no old files). If you see a empty disk with a directory Backups.backupdb - that implies that the format was successful and the Mac is ready to write files which will over-write free space and in time ensure that none of the old pictures are recoverable.
I'd say - get a hold of someone that has done drive recovery before and/or get a quote from a company like Kroll Ontrack so that you know what your options are.
The more data that gets written to the drive and the more uncertain you are about exactly what happened will increase the work needed to recover the files, but I would guess you only have to pay for software and time to get some if not most of the pictures back. If you performed the recovery yourself, you'll need a third hard drive to store the recovered files (or store them on the Mac if there is space) and the $99 software which you might be able to download and try for free.
If these files were PC formatted, you might have better luck with a PC recovery software, but I've used Data Rescue on many PC drives in the past and had good luck and not needed to send the drive out for professional recovery - so you may not need anything other than what I have listed above.