Photos uploaded per month
I can't recall what happened around 2010, whether it was a change of forum software or hosting provider which made things easier to upload photos in bulk - or just more enthusiastic members joining and uploading photos!
That all lead to the "Great Purge" of 2010 which saw over 25,000 photos deleted due to an emergency situation with disk space restrictions (back then, disk space for hosting providers was incredibly expensive). I still regret that event to this day - you can read more about it here:
Missing Photos
Note that those photos that were purged do not show in the chart - so the data in the chart before October 2010 doesn't reflect the actual number of photos that were uploaded.
So the large spike in late 2010 will be from people busily uploading new photos to replace the ones that were removed (once the hosting issues had been resolved).
FWIW, I'm now running ZooChat on a server that is literally 4x more expensive than it requires for the traffic we get, simply so that I have enough disk space for all the photos. I'm hoping that the upcoming new release of the forum software later this year will make it easy to have photos automatically migrated to bulk storage such as Amazon S3, which will remove all space limitations and it won't matter (as much) how many photos we upload.
Ironically, about the same time I expect that solution from our software provider, my hosting provider is likely to offer a different solution - so I may well be spoilt for choice.