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(in which case someone might be forced to look it up), you become very famous (in which case it will be an interesting historical document), or you do something so weird in it that people pass around copies for the amusement or shock value.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".
Typically, you want as few "personal touches" on your dissertation as possible, as each one will increase time between your submitting it and the university accepting it. There are many places to assert your individuality within the research community (your speaking style, your writing style in your publications, the types of research problems you tackle, your mentoring style), but this is not one of them.
We have a very similar practice here, although I think our booklets are usually a little bigger (B5-sized instead of A5). Looking at the local university press guidelines for dissertation writers (in Finnish, sorry; there's an English version, but I couldn't find anything similar there), they say (translation mine):
"The cover should describe the content of the thesis and invite one to read it. The cover may either be purely textual, or it may include pictorial elements. For textual covers, it is worth paying attention to typography and text layout.
If you want a picture on the cover, it can be either a picture from your thesis, a picture chosen by yourself or an illustration selected by our graphic designers. [...]
If the thesis is published as part of a series, however, the cover must follow the graphical guidelines of the series."
Putting a picture of yourself on the cover of your thesis seems a little unusual, because the cover is normally supposed to illustrate the thesis, and the person presumably didn't write her thesis about herself. However, as long as there's nothing clearly inappropriate about the cover (I assume it was a picture of her face...), it seems to be her choice to make.
Does that mean you should follow her example? Probably not, at least unless you believe there's something for you to gain by doing that.
Does it mean you should try to come up with a nice, elegant and unique design, illustrative of your chosen topic, for your thesis cover? Certainly, if you can! (Having friends or relatives who happen to be artists or graphic designers can help here.) At least, it won't hurt, and it just might increase the chances of someone picking it up and reading it.
Does it mean you should stress about it? Certainly not — "a molecule, a graph, a microscopy shot etc." are all perfectly good choices for a thesis cover, and a text-only cover is a perfectly fine choice too.