It looks like you didn't quite remove all the background before saving to a png-24 or any format that supports transparency.
How does it look in photoshop if you put a background behind the building?
I tried putting a black background behind it and it looks fine...
Take a look in the layers menu way down at the bottom. You should find Matting>Defringe or Remove Black Matte or remove white matte. One of these should get rid of that halo.
Also save for web can introduce a halo, the color would be the background color. But its mainly noticable when save as a gif.
I am not positive, but I think the intoduction of the halo, is dependant on how the transparency is used. There are two types, straight and multiplied. I think the multiplied is the cuprit because the transparency is part of the RGB data, where the straight keeps the transparency seperate.
I am probably over simplifing the differences, but hopefully it will point you in the right direction.
In the save for web dialog change the matte from a color (white is default I believe) to none. Hopefully that will fix it.