First of all, so sorry for taking so long to get back to you. I've had a very busy last couple of weeks.
I currently have been storing all of the photos here: https://cloud.gnome.org/index.php/s/bGcGPttHVDcw0dQ
The photos are each named with the licensing information to make sure that everything is accounted for and useable. I think it'd be a great idea to have another place to store the licensing information. Maybe in an EtherCalc?
As for my process in gathering these photos... I started off with a list of people that typically take photos at events, and sent them emails or private messages on social media (i.e. Facebook & Flickr). The message was something like this: