You can modify the pictures posted from other member without having modpower or permission to do this. To verify it: just post an reply to any gallery thread. Then edit your own reply. In this you get the filemanager and can post own pictures. Submit it and your new thumb will display instead of the original thumb.
/update: if this happen to your gallery you can fix it: just say edit on the first post, go to picture manager, change nothing and say your are finish it. Submit the changes to the post and the original thumb will be rebuild.