Seems like it is no easy way to display all the media files under a wikimedia category -- for example if someone wants a picture of a library, he or she will need to go into each sub-category under "Libraries":
http://commons.wikimedia.org/wiki/Category:Libraries
While Wikimedia is not yet the most popular stock photo source, IMO having this flattening functionality would be useful to those who are looking for stock photos.
Rayson