OK, the general consensus on UI's 49 and 50 is that I may have taken too perverse a pleasure in coming up with tough ones. I'm still not giving up the game, but more hints: the building seen in UI 49 is used for exterior shots on one of my favorite network teevee programs. (But the building is not located in the city where the show takes place.) Identifying the show earns partial credit, but actually coming up with the name of the building (yes, you can use Google, and how would I know, anyway?) gets full credit.
UI 50, on the other hand, may have been seen by people who have visited Walt Disney World. No more than that.
As for UI 51, everybody's right: that there is the famed opera house in Sydney, Australia.
And now for what I'm sure is another easy one:
Where are we? Rot-13 your guesses, please!