I found this neat photo today. U.S. Capitol & Graf Zepplin (source: Library of Congress). I was curious to see if I could identify the specific zeppelin pictured here, and I think I might have...
If you access the TIFF file available at LOC and zoom in on the side of the zeppelin, you can see that there are some letters visible (upper left). Auto-contrast on Photoshop makes it a little easier to see these:
Does that read D LZ127? Could it be the LZ 127 Graf Zeppelin which was registered as D-LZ 127?
The exact date of the photograph is not recorded, but the LZ 127 Graf Zeppelin did pass over DC at least once. Wikipedia's article explains that on her first transcontinental flight, she "passed over Washington, D.C., at 12:20 PM." (The New York Times, October 16, 1928.)