For many, I used the Wordpress Media resizing tool. At other times, I've used a resizer tool I have on my desktop. I have used both simply depending on what I've found most convenient at the time.
For the one you provided, I simply downloaded it to my computer, then uploaded it to the media area, then went to the option for that page I linked to to upload the header image and choose the picture you gave me.
As far as the CSS on the homepage, someone else did that for me to get the photo to show as well as it does now on that page, though I'd rather have more skyline showing. It really doesn't seem to matter WHAT I want the picture to show, lol, it never really ever shows ANY picture I put on those header images the way I want them to or expect them to according to how the photo displays, unless they are enormous. Even then, there have been a few that would show only the top part of the photo, or just the bottom part, etc.