Graphics/Design aren't really areas where there is much research going on. The real research usually happens in Computer Science /Engineering departments, and while some applications might have huge impact in the graphics industry, I don't think there are any PhD programs (in HK) focussing on such topics.
What I do know is that CUHKs engineering faculty has some professors who do really interesting stuff in image processing. I remember a demonstration where they demonstrated a pattern recognition program. You would feed old mangas into the program - those that had only every other page printed in colour, while the others used BW patterns. It would then automatically create a coloured version of the entire comic, including shadings and such. But if you start in that faculty as a PhD student, you'll go through at least two semesters of tough math - particular Fourier transforms in all varieties - before you ever get anywhere near anything related to graphics.
Generally speaking, I'd rank the locals schools UST>CUHK>CityU>HKU for CS/engineering. Of course this is very subjective and all I know is hearsay from other graduate students, but at UST you're the core of the university, while the focus of HKU, afaik, is more tilted towards Medicine, Law and Bioscience.