Convert an image to Audio
so that the image shows up in the
spectrogram of the audio file

Use audacity to view image.

Use this website to convert an image to sound
https://ojack.xyz/PIXELSYNTH/

Create Spectrogram
ffmpeg -i audio.mp3 -lavfi showspectrumpic=s=800x400:mode=separate spectrogram.png

or if audio is in a wav format already
sox audio.wav -n spectrogram -o spectrogram.png