I think it's possible for Shiina to do this; some things look a lot harder than it is. I haven't done it myself, but I would just think she would just have to take a sample that would blend in the background very nicely, and gradually shift the tone up. Then take the same sample, and gradually shift the tone down for the same amount of time. Lastly, mix the two together.
Image stenography (hiding an image within another image) is a lot more complicated involving math but is its the same concept, and I was able to do it without former expertise in image processing.
She probably was influenced to do it by someone else though..