You can view sources as thumbnail pictures in List View—this is especially useful when you are working with pictures and videos.
To display thumbnails:
Click the Sources button in Navigation View.
Click the required source folder.
Click in the List View.
On the View menu, click List View.
Select the required thumbnail options.
For video sources, you can assign a specific frame to be viewed or displayed as a thumbnail in List View:
Open the video source in Detail View.
In the timeline, drag the playhead to the desired frame.
In the Media menu, click Assign Frame as Thumbnail.