[JD557]

Disclaimer: I'm probably wrong about this, this is just how I believe "Neural style transfer" works. I never tried this out and there's probably a lot of problems with my explanation.

I believe that this is done using Restricted Boltzman Machines[1] trained with the stylised image.

Think of it as a network that receives an image on the input layer, sends it to one or more hidden layers with less nodes (like an auto-encoder), and then tries to reconstruct the image on the output nodes. This is like a lossy compressor-decompressor overfitted to the stylized image.

Now, just pass the real image as an input to your network and the output should be a stylized version of the input.

[1]http://deeplearning4j.org/restrictedboltzmannmachine