You mention you want to use these images as reference. If you grabbed these images, used live trace on them, and created a collage, you'd most likely be infringing copyright. As Scott mentions, the lighting, angle, exposure, etc are copyrighted.
I don't think (and this is my personal opinion, in these matters the only way to make sure is to ask a lawyer) that you are infringing it by using an image as a reference, to draw for example a silhouette. As long as you are the one creating the drawing.
Unfortunately, only you will know how much of someone else's material you used as inspiration. The extremes are easy, if you only modify the image a little and call it your own it is indeed infringement (and highly unethical). It gets progressively gray from there. I like sketching buildings and cities, and lots of times I go to the library and use books as references. I would never 'copy' a picture of, say, a person. But most references I use are frontal photos of buildings, I combine architectural elements. I don't think I'm infringing copyright.
If you have the chance to take the pictures yourself, you should definitely go for it, as you will know which angles you need the most to create a consistent skyline. If you are going to use references, I'd try to always make sure they are no more than references.