Generally zenphoto should display them correct, but if your filenames included ÖÄÜ then your host's server must support that (I am german and all the hosts I know here don't!). I recommend uploading images without any special characters and rename the images in the admin if those characters are needed. That makes the live easier..:-)
And of course your MySQL-Database must have the right encoding too.

If I have scandinavian letters in my filename, everything works out great. But if I have scandinavian letters in file-info (description) or keywords (tags), they are shown as question marks.

It's not because of my server provider, and it's propably not because of mysql either, though I don't understand how I could change mysql to utf-8 (or to anything).

The idea of changing all question marks in admin is not an option, because in Finnish language ä and ö are almoust as common as i or e in English, and there is usually a lot of text in fileinfos.

Anyway, thanks for one of the greatest software ever!

Ok, then I got you wrong, of course changing that manually is not an option. I supposed you have the 1.1.2 release running? Have you maybe altered a theme and maybe by accident saved the files with the wrong encoding? I did this mistake myself several times...If that is so all utf-8 encoding within zp, your server etc doesn't help. Please check this. If that is not the case, we might have really a problem...

I have version 1.1.2. I checked zp-core and default-theme directory. There was some files which wasnt utf-8, but after I had saved them as utf-8 file-infos and keywords were still question marks. So there is still a problem to solve.

I had a lot of utf-8 problems too. Here is what i do to avoid it:

1. set utf-8 to false.
2. refresh the zenphoto-database & metadata
3. set utf-8 to true again.

This works for me.