Aphex Twin, Nine Inch Nails, Disasterpeace and in all likelihood, many other musicians have been hiding images in their songs for years.
These Easter eggs can be found using something called a spectrograph, an application or instrument that creates visual renderings of audio signals. Other programs out there, like this one, or this one, do the opposite. Pug in an image and they’ll turn it into a sound file. Mix it into your song at an inaudible level, and BAM, you have trippy subliminal sound-images. Here’s a video tutorial.
At this point, no word yet of whether or not you can use these messages for mind control — or album sales. Just a curiosity.