By
default, the first frame of a clip appears in the thumbnail viewer
and in other places in the project where the thumbnail is displayed.
You can override the default thumbnail by designating any clip frame
as a poster frame.
Select the clip in a Project panel.
Press the Play button or
drag the play slider on the thumbnail viewer in the upper left corner
of the Project panel until the frame you want is displayed.
Click the Set Poster Frame button .
You can also set the poster frame by right‑clicking
(Windows) or Control-clicking (Mac OS) the thumbnail viewer and
choosing Set Poster Frame.