Do you have to convert it into an image? What is it about classifiers that require image input? I've always found it very cool that audio compression and image compression end up using similar frequency-space techniques sometimes.
You don't have to. It's just that there are a lot of good image classification network architectures that exist already so you can more or less grab a good network off-the-shelf, then give it more targeted training examples so that it performs well in your specific use-case.
I guess it comes down to the fact that a lot of these signal processing techniques involve convolution.
