During a running contest the name of the user that has uploaded the picture is hidden. This is working as it should.
But, if I start a search on pictures on a particular user also the pictures that this user has uploaded to a contest are showing up. The name is still not shown under the picture but as the search has been performed on a single user name the owner of the contest picture is disclosed this way.
Solution found in this thread: http://www.flying-bits.org/viewtopic.php?f=101&t=3948&p=17614#p17614