Merkouris in Greece experimented with the decoding of an image from the second audio harmonic of a VOA Radiogram transmission.

Roger in Germany followed up by decoding from the third audio harmonic of The Mighty KBC’s transmission on 9400 kHz (via Bulgaria.)

At that point, I wrote, “And I think that is a far as we will be able to go with harmonic image decoding!”

I was wrong.

Roger, taking advantage of a strong KBC signal on 9400 kHz, was able to decode MFSK16 text from the fourth audio harmonic …

http://www.rhci-online.net/radiogram/VoA_Radiogram_2017-05-27.htm#decoding_4th_harmonic

But perhaps now we have reached our limit.  Roger wrote, “I also tried the 5th harmonic, but the result was not usable.”