How are you curating these images? Which is to say, how are you ensuring that I see a really great photo of the Azure Window[0] (which happened to be posted two weeks ago) rather than whichever random one Ig turns up first?
I had a semi-similar idea (but focused on graffiti) but all the restrictions in their API terms were sort of a turn off.
[0] http://instagram.com/p/itwBoKwBnp/