Commissioning artist to make a figure
It's your thesis, and so long as the work --- including the cover ---conforms to the requirements set down by your institution, then I see no problem with it. Yes, you might upset some of the more conservative of your colleagues, but if you continue in research no doubt you will in future perturb the status quo again.

The only concern I have is whether such an unconventional decoration would influence a grader. However, I suspect you are considering the final version of the thesis, after any corrections set down by your examiners, so this isn't an issue.

Naturally, you should give credit to your artist.