You can manually generate thumbnail for each video by executing the following steps:
1) Go to Content ->Videos->Edit Video, at Thumbnail section click "Change" button to open the "Generate thumbnails" dialog .
2) On the "Generate thumbnails" dialog click on the "Get the currently displayed frame" several times in order to generate several thumbnails.
3) Select the best thumbnail that fit your needs and click button "Save".
4) Publish the video.
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