How much of a "personal touch" on dissertation is OK?
This probably depends a lot on how things work at your university.  In the cases I'm familiar with (U.S. math departments), there are only four types of people who have an opinion about your dissertation:


You.
Your thesis commitee, particularly your advisor.
The bureaucrat who measures the margins and decides whether your paper is fancy enough.
Family members and friends who will want to take a look at the nicely bound volume and make polite comments.


Nobody else will pay any attention to your dissertation, unless you do an inadequate job of publishing its contents (in which case someone might be forced to look it up), you become very famous (in which case it will be an interesting historical document), or you do something so weird in it that people pass around copies for the amusement or shock value.

The painting on the cover might fall into the last category if it looks pompous or strange enough.  However, most personal touches are not a problem if the four groups mentioned above don't care.  Furthermore, the bureaucrat won't care unless you violate a known rule or do something so awful that they feel the need to dig up an obscure rule, and your family doesn't get a vote (you'll just be embarrassed if they think you've done something silly).

So the short answer is "ask your advisor".