Have been fiddling with the process for a while but here is a top tip for getting digital images onto some headphones... inkjet water slide decal paper.
Model makers use it for making custom decals and signs for model railways, planes etc - you can print the image on a standard inkjet printer - give it a coat of clear acrylic to stabilize it - then when you soak it in water the top layer with the ink on can be slid off onto your project. I found painting the headphones white first and then lacquering it to give a nice smooth surface first is pretty important then print and go.
Once you have the decal on the earcup make sure you work any air bubbles out to the edge and then let it dry fully. once dry give it a final coat of clear lacquer.
Here are some DT770s I recently did using this technique - it then blended the image in with an airbrush but that is not totally necessary if you chose your design carefully.