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.

I see no problem when it comes to presentations (eg on the first slide to give the general idea). As long as the "corporate design" of the academic institution is still recognizable. Otherwise slide 2 or 3 would work.

Thesis cover might work as well, as long as it's within regulations (eg cover not nitpickingly specified). And as long as there isn't even a hint that you want to put style over substance. Also, conform when it comes to the spine. If the thesis is in a library as physical copy, it's the spine that counts (and should look the same way as the rest). The cover is invisible.

As for figures to actually convey data in the thesis, I'd stick to the convention. There are reasons -- beyond a departments preference -- why graphs look the way they do. Any fluff is clutter that has no place in a scientific diagram. The primary aim is to argue, not to entertain.

But yeah, go for the cover. Reminds me of Randy Pausch's ("Last Lecture") story about the color image in a article ("Are there allowed to do that?!?!").