http://thesecret.pbwiki.com/Theories
National Register of Historic Places Theory
The images for both of the found casques, IMAGE 4 and IMAGE 5 included a landmark building near the burial site. Both of these landmark buildings were on the National Register of Historic Places
IMAGE 4 – Cleveland Terminal Tower
http://en.wikipedia.org/wiki/Cleveland_Union_Terminal
http://en.wikipedia.org/wiki/Terminal_Tower
http://www.nationalregisterofhistoricplaces.com/OH/Cuyahoga/state7.html
Union Terminal Group *** (added 1976 – Building – #76001405)
IMAGE 5 – Chicago Water Tower
http://www.nationalregisterofhistoricplaces.com/il/Cook/state2.html
Chicago Avenue Water Tower and Pumping Station ** (added 1975 – District – #75000644)
Also known as Old Chicago Water Tower District
Both sides of N. Michigan Ave. between E. Chicago and E. Pearson Sts., Chicago
http://en.wikipedia.org/wiki/List_of_Registered_Historic_Places_in_Cook_County%2C_Illinois *
http://en.wikipedia.org/wiki/Chicago_Water_Tower 
NRHP THEORY: Perhaps the other sites include buildings that were on the National Register in 1989.  Maybe this was intentional by the author, or perhaps the author simply picked significant buildings that tended to make the NRHP list.  Either way, it may be worth reading through the list and looking for buildings that may be hidden in the images.
Check the list here:
http://en.wikipedia.org/wiki/List_of_Na … es_entries
IMAGE 7 – NEW ORLEANS?
http://www.nationalregisterofhistoricplaces.com/LA/Orleans/state.html
http://en.wikipedia.org/wiki/Louis_Armstrong_Park
http://www.nps.gov/archive/jazz/Armstrong%20Park.htm
Just a theory… please let me know how it applies (or doesn’t) to your favorite image.
AnotherDoth

It might be the case for some casques, but not likely all.  BP confirmed that one casque was in the Childrens’ Zoo at Hermann Park in Houston — and a quick glance at the list turns up nothing for that location in the NRHP.
If it doesn’t apply across the board, it’s probably not going to be useful for confirming locations, but it could be a good starting place.

also important to note they would just be city confirmers.
in that The water tower and the terminal tower are both in the cities but not at all near the casque locations.
still all clues are worth looking at