copyright and ip

Copyright is one form of intellectual property. (A trademark, for example, is another.)

Is a work of art on the street someone's IP? Probably, yes. It's the intellectual property of the artist who created it in the first instance, but he may have sold or transferred his IP in the work to someone else, so it could now be someone else's IP. It doesn't cease to be intellectual property simply because the work of art is in the street, any more than your car ceases to be your car when you park it in the street, as opposed to your own front drive.

Can you photograph it without his permission, though? Ah, that's complicated. On the one hand, copyright is (as the name suggests) first and foremost the right to make copies. You generally can't photograph (without permission) a work which is the copyright of someone else. On the other hand, you generally can take photographs in public places, and there's an argument that by displaying his work in the street the IP owner has licensed people to photograph it.

I think a lot is going to depend on the context. If your photograph is a streetscape in which the work happens to appear because it was in the street at the time, that's one thing. If your photograph is, basically, a photograph of the artwork, that might be another.