How much of a "personal touch" on dissertation is OK? I don't know if this is the case at other institutions but at my university dissertations are published as A5-size books/booklets. There are some regulations on how the formatting is supposed to be but it's pretty relaxed otherwise. Common practice is that the PhD candidate designs the cover themselves. 

Typically these covers feature an image relating to the work the person has done in the lab; a molecule, a graph, a microscopy shot etc. Recently a colleague of mine put a painting of herself on the cover. Just for the sake of clarity, not a painting she's done, but rather a painting of her made by someone else. This has raised some discussion among the younger students, such as myself. Thus I wonder, how much of a personal touch is OK really? Would something like I described above be seen as an ego-thing by a potential employer? I am of the opinion that it might be seen as a lack of modesty or "self-distance" (quotes are there as I am not sure if this expression exists in english).

Am I being too critical? As it's a multi-cultural international group of academics here, I wonder if there is a consensus regarding such "customization" of the dissertation.
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(in which case someone might be forced to look it up), you become very famous (in which case it will be an interesting historical document), or you do something so weird in it that people pass around copies for the amusement or shock value.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".