How do I move thumbnail photos within an event?
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You can't do this is an Event. Events are a very basic time and date based organisation. To change the order of things in an Event, change the time relative to the pic you want to precede.
Or... Make an album of the Event, then you can organise anything anyway you want. To do this just drag the Event to the Albums heading and it will make an album of the same name. As Albums only reference files in the Library they take up no extra disk space and have much better organisation possibilities.