http://www.nicolasfournel.com/audiopaint.htm (Audiopaint)
This would be used to generate the spectrograph images, be careful, as incorrect settings may result in damage to your equipment, and/or hearing.
(In other words, keep the volume low, just to be safe) If converted properly, the image will sound like a "Swoosh"

http://www.dxatlas.com/sstvtools/ (SSTV tools)
A bit of a finicky program, but rather easy once you get used to it, this can convert a small .BMP file to an SSTV signal, with any of the known signal broadcast methods.
The resulting sound file should be a series of beeps, squeals, and static.
Even common broadcast errors can be emulated with impressive accuracy

http://www.dxatlas.com/morsemidi/ (Morse Midi)
A rather simple system, You input text, and it outputs an MIDI in morse code.
The only problem area is getting the WPM right.
I suggest soundfonting the resulting MIDI for better compatibility.

http://www.electronics-lab.com/downloads/pc/003/ (Spectrogram)
This is a variable sound spectrum analyzer, or spectrogram.
This will continuously scan all input audio and represent it as a series of dashes and lines, within the given threshold.
An image that has been converted into an audio file using Audiopaint can be decoded with this.
(Try it on that Lavender town syndrome creepypasta track, you might be surprised what will actually show up)

http://rx-sstv.software.informer.com/ (RX-SSTV)
When playing an SSTV sound file, this will decode it into an image.

http://download.cnet.com/CW-Decoder/300 ... 25058.html (CW Decoder)
This is the Morse code decoder, there are actually a bunch of these online, but this one had proven to be the easiest to set up.
It will always mistype at the start, but after a bit, it will start spelling correctly.