Found a solution, i'll post it here in case someone else runs into this:
It cant be done with Snow Leopard tools. It used to be a feature in QuickTime, but was removed from Quicktime X.
What you do is install QuickTime 7.0.
Cmd-i on the video-icon to open the info-pane. Then Open the movie in QT 7.0, scroll to which-ever frame you like, and drag this image from the QT-window to the info-pane on the icon top-left corner. That's it.
I cant understand why apple would remove this feature from QT X??