Song with the hidden picture · uncompressed
After MP3 encoding and decoding
The MP3 is the file made by the codec test, at the bitrate chosen there; save it once the test shows the picture survives. Anyone can see the picture in a spectrogram viewer, such as the Reveal tab or Audacity.
How the picture is drawn
Each pixel column becomes a moment of the song and each row a frequency inside the band. Brightness sets the amplitude, phases are random, and an inverse short-time Fourier transform turns the picture into a faint noise that is added to the music.
How audibility is estimated
Within each critical band the picture's energy is weighted by the threshold in quiet (Terhardt, 1979) at the chosen listening level. It counts as audible when it exceeds that threshold and is no more than 6 dB below the music in the same band. Young ears can be about 10 dB more sensitive near 14–16 kHz than the formula says, so keep a margin.
Why MP3 erases it
Encoders discard what the ear cannot hear. At 128 kbps LAME low-passes around 16–17 kHz, so a picture above that line disappears. Just below the line it is kept, but only if it is strong enough for the encoder to spend bits on it. That is the trade-off you are tuning.