Update: changing image size through the media center portion of wordpress seems to work, but not via the ‘insert image’-applet when editing posts (as mentioned above, greyed out). Is this on purpose?
Ok, uhm, that was kinda quick… but I had a hunch:
I switched back to php 5.2 and now it seems to work fine.
So this is now a bug-report: The Plugin is incompatible with PHP 5.3
