You need Eye of the Grey Ones to interact with the fountain. It is a plot item dropped by Grey Sisters. It's best to visit Great Hall of Propontis before you give the Eye to Medea, but it can be farmed again later if need be.
GoG Galaxy should store your screenshots at:
I'm afraid it's not customizable.
Steam screenshot path can be found through:
View > Screenshot > Select game > Show on disk
I don't use Steam, but I believe this path can't be changed either.