I heard you were thinking of Grant Park for your pic.
Not to pipe up and run my mouth (I've NEVER done that... right?!) and try and change things at the last minute...
BUT
What about the art meuseum for a location?
There was an international ad run by Porsche with a picture of a 911 in front of the art meuseum (I saw it in AUTOCAR - a brit magazine) and it looked really classy. I've seen a lot of private shots of cars there, too - all looked pretty good. You gals could probably get a prime location with your bikes (easier to drive on sidewalks than cars).
Either that, or in front of the old Pabst brewery
Just my 2 cents...