Typically, the New Orleans Times-Picayune has great coverage. They have a bunch of links to sets of photos from the disaster.
This one is a terrific montage of photos that covers the first week of the disaster.
The rest of the featured collections are self-explanatory:
A Ravaged City
Destruction in Slidell
Crews Find Bodies
This page is the directory of all the Times-Pic photo collections from Katrina.
This page may be the master directory for the NOLA.com images. It includes galleries of amateur photos as well.