This is one of a series of questions submitted by attendees of the Copyright, Common Mistakes and Myths Webinar and answered by Laura N. Gasaway, author of Pocket Copyright Guide for Publishers.
Answer: Courts are struggling with this right now, so there is no definitive quantity, etc. The best way I know to describe the amount of transformation that must take place for a photograph is a whole lot! Almost so much that it no longer resembles the original image. So, cropping the photo is not enough or using only a portion, such as the central figure or central building, etc., would not be enough.
Pocket Copyright Guide for Publishers by Laura N. Gasaway and edited by Iris Hanney contains information vital to the publishing community.