Sorry to butt in...
....I just muttered "sashin daijobu des ka?" and
took 2 pictures.
What does that mean exactly? From what I've gathered from a hobby
of mine - anime - that "daijobu des ka" means something like
"everything all right?"

But I would bet that I'm wrong..
In some some contexts, that is exactly what "daijobu desu ka" means. In other cases, one can use it to ask permission, as in "it is OK to take a pic?" Remove the "ka" (the Japanese version of the question mark), and it becomes "its fine [OK, cool]".
Once I was trekking with a couple of friends in Lapland in a quite
popular part of woods and the only people we saw during that 4 day
hike was a group of Japanese men and women. We met in a cottage
when they came there and we were just leaving. I had my p&s camera
with me, which I knew wouldn't do very well indoors so I dumped the
idea to do portraits of them and send them the photos afterwards as
a gift... But I've regretted my cowardice many, many times and
wouldn't want to regret a possibility like that again!
The Japanese are often BIG on taking portraits to commemorate an event, so it is usually Ok to ask. In some cases, Japanese tourists will go to see some unique and very photogenic place hefting a camera, but in the end only take a pic of their friends with the place as a back drop.
Now, I'd like to know some phrases with which you could politely
introduce yourself and ask a permission to photograph someone in
Japanese

Just the basics, please. First of all, what does that
mean what you said to the young lady?
Ian's initial phrase will work quite well. Other options include
"Sumimasen ga, shashin wo totte ii deshou ka" (Excuse me, may I take a picture).
And after you are done,
"arigatou gozaimasu" (thank you)
"sumimasen" (exuse me for bothering you)
If some one really goes out of their way to help you (like taking you on a tour, or something like that)
"taihen osewa ni narimashita" (you have helped me a great deal) is a rather polite phrase, but you will get more mileage from the two phrases above.
Note on pronunciation, the double vowels (ii, ou) represent slightly elongated sounds, not changes in sounds. For example, both "i" and "ii" sound "ee" in the word "bee", but the "ii" held just a bit longer. "To" and "tou" are both pronounced as "oe" in the English "toe," not like the pronoun "to" in "going to the store."
As for the personal introduction, that really depends on what you would want to say.
I'd be very happy to hear any suggestions - especially from someone
who speaks Japanese

Thanks in advance!
I could use your thoughts on this image. What works
best? The B&W version or Color. What about critiquing the image
in general. How is the contrast, subject, comp?
The colors are so beautiful and emphasize the face in such a way I
wouldn't even consider the BW version..

The BW one would need a
lot of burning in my opinion, or maybe a contrast boost with an
S-curve. Have you tried Greg Gorman's tricks in BW-conversion? I'll
post a link:
http://www.gormanphotography.com/gorman.html
Just press the 'learn' button.. On the other hand, I have it as a
photoshop action - I'm not sure where I got it, though

Must've
been via the forum here..
Janne Mankila