Show the Movie track in GarageBand on Mac

The Movie track displays frames of the movie as thumbnails. The number of frames displayed depends on the zoom level of the Tracks area. All frames are aligned left except the last frame, which is aligned to the right. This guarantees that you can always see the first and last frames of video, independent of the current zoom level. The left margin of a frame (except the last frame) is always displayed at the exact time position of the frame.

You can set the resolution of the thumbnail movie frames in the Advanced pane of GarageBand settings.