It's remarkably hard to find images of what it used to look like. I've seen some before. Here's what one section looked like in the 1870s when the railroad run underneath it:

http://www.iridetheharlemline.com/wp-content/uploads/2010/05...

Here's park avenue in the 1920s:

http://screencast.com/t/7GRMhOzq

Park Avenue (used to be 4th Avenue) has probably changed more than most Avenues in NYC. Here are some images of when railroads ran down it.

http://ephemeralnewyork.wordpress.com/2013/01/28/three-views...

You may also enjoy seeing these photos of the NYC mansions from back in the day:

http://ny.curbed.com/archives/2012/02/09/looking_back_at_man...