This can be done in QuickTime Player 7 (although you may need the Pro version). See the following Apple article:
Open the movie in QuickTime Player 7 then move the playhead to the desired frame. From the menu, select View>Select Poster Frame. Save the movie and your new poster frame will be set.
That's strange. Before posting my reply I tested the procedure using a .mov file which was encoded as Apple Intermediate Codec (AIC). That worked fine and the poster frame I had selected was visible in the file's thumbnail preview in Finder.
I subsequently tried updating a couple of other movie files. To my surprise, files encoded as H.264 in either .mov or .m4v did not show the new poster frame. In QuickTime Player 7, if I selected View>Go to Poster Frame, the correct frame was displayed. However, the only way I could get the poster frame to "stick" was to export the file again with QuickTime Player 7. Simply saving the file did not save the new poster frame.
Here is the Help item from QuickTime Player 7 Help:
Setting a Movie's Poster Frame
A "poster frame" is a still image of a movie that represents the movie in places like the Finder. The default poster frame is the first frame in the movie. You can change the poster frame.
To change a movie's poster frame:
- In QuickTime Player 7, drag the playhead (or use the arrow keys to move the playhead) to the desired frame.
- Choose View > Set Poster Frame.