the server is our production server, and has the recommended settings.
tried some more. I am able to upload and change pictures on 1.5.4 RC1 and 1.5.4 beta on the same server, which both were fresh installs.
php is compiled with GD, and it was a jpg.
the content directory is not in the atutor directory, but in a different directory at root level on the server (same level as the atutor dir), and is called atutor-innhold/
within that directory there was no folder called profile_pictures, but I created it and chmodded it to 755 (and 777) - still no luck though.
the tmp/ directory is most probably fine, as all other uploads work.